diff --git a/CHANGELOG.md b/CHANGELOG.md
index 514c637..9c22c21 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,7 @@ All notable changes to this project will be documented in this file.
 - Add an `encoding` kwarg on `MetricReader.open` and `MetricWriter.open` (#43)
 - Transparent gzip/bz2/xz support via `xopen` (#44)
 - Infer the delimiter from the file extension in `MetricReader.open`/`MetricWriter.open`; pass `delimiter=` to override (#61)
+- Add append mode to `MetricWriter.open()` via `mode="a"`; appends to a file that has a matching header or that is headerless data validated against the model, raising only when the first row is neither (#47)
 
 ### Changed
 
diff --git a/fgmetric/metric_writer.py b/fgmetric/metric_writer.py
index 98ef4a0..78ec497 100644
--- a/fgmetric/metric_writer.py
+++ b/fgmetric/metric_writer.py
@@ -1,11 +1,14 @@
 from collections.abc import Iterable
 from collections.abc import Iterator
 from contextlib import contextmanager
+from csv import DictReader
 from csv import DictWriter
 from pathlib import Path
+from typing import Literal
 from typing import Self
 from typing import TextIO
 
+from pydantic import ValidationError
 from xopen import xopen
 
 from fgmetric._delimiter import infer_delimiter
@@ -13,6 +16,111 @@
 from fgmetric.metric import Metric
 
 
+def _read_first_row(
+    path: Path | str,
+    delimiter: str,
+    encoding: str,
+) -> list[str] | None:
+    """
+    Return the first row of an existing, non-empty file as a list of cells, or `None`.
+
+    Returns `None` when the file is missing or empty. Otherwise the first row is parsed with
+    `csv.DictReader` — the symmetric counterpart to the `DictWriter` used to write metrics — so it
+    is interpreted exactly as it was written. The row may be a header or a data record; the caller
+    decides which by comparing it to the expected column names and validating it against the model.
+
+    Args:
+        path: Filesystem path to inspect.
+        delimiter: The delimiter used to parse the row.
+        encoding: The text encoding used to decode the file.
+
+    Returns:
+        The parsed first row, or `None` if the file is missing or empty.
+    """
+    file = Path(path)
+    # A zero-byte file is not a valid compressed stream; checking the size first avoids the
+    # bz2/xz decompressors raising EOFError when xopen tries to read an empty `.bz2`/`.xz`.
+    if not file.exists() or file.stat().st_size == 0:
+        return None
+    with xopen(path, mode="rt", encoding=encoding) as handle:
+        fieldnames = DictReader(handle, delimiter=delimiter).fieldnames
+        return None if fieldnames is None else list(fieldnames)
+
+
+def _row_validates_as_record(
+    metric_class: type[Metric],
+    fieldnames: list[str],
+    row: list[str],
+) -> bool:
+    """
+    Whether `row`, zipped onto `fieldnames`, validates as a record of `metric_class`.
+
+    The cells are mapped to `fieldnames` and validated with `model_validate`, the same path
+    `MetricReader` uses to parse a row. A row with the wrong number of cells cannot be a record,
+    so it is rejected without attempting validation.
+
+    Args:
+        metric_class: Metric class to validate against.
+        fieldnames: The expected column names (the metric's header fields).
+        row: The parsed cells of the row to validate.
+
+    Returns:
+        `True` if the row validates as a record, `False` otherwise.
+    """
+    if len(row) != len(fieldnames):
+        return False
+    try:
+        metric_class.model_validate(dict(zip(fieldnames, row, strict=True)))
+    except ValidationError:
+        return False
+    return True
+
+
+def _append_needs_header(
+    metric_class: type[Metric],
+    path: Path | str,
+    delimiter: str,
+    encoding: str,
+) -> bool:
+    """
+    Inspect a file opened for appending and report whether to write a header.
+
+    Returns `True` when `path` is missing or empty — append-or-create writes a fresh header.
+    When `path` already has content, its first row is accepted if it either matches the expected
+    header or validates as a record of the metric class (a headerless data file); in both cases
+    `False` is returned so no header is written into the middle of the file. A `ValueError` is
+    raised only when the first row is neither.
+
+    Args:
+        metric_class: Metric class whose header/records the file must be consistent with.
+        path: Filesystem path being opened for appending.
+        delimiter: The delimiter used to parse the first row.
+        encoding: The text encoding used to decode the file.
+
+    Returns:
+        Whether a header row must be written.
+
+    Raises:
+        ValueError: If `path` is non-empty and its first row is neither the expected header nor a
+            valid record.
+    """
+    first_row = _read_first_row(path, delimiter, encoding)
+    if first_row is None:
+        return True
+
+    expected = metric_class._header_fieldnames()
+    # A matching header, or a headerless file whose first row validates as a record: append after
+    # the existing content without writing a header.
+    if first_row == expected or _row_validates_as_record(metric_class, expected, first_row):
+        return False
+
+    raise ValueError(
+        f"First row of {path} is neither a {metric_class.__name__} header nor a valid record.\n"
+        f"  expected header: {expected}\n"
+        f"  found:           {first_row}"
+    )
+
+
 class MetricWriter[T: Metric]:
     """
     Write `Metric` instances to a text IO sink.
@@ -32,15 +140,21 @@ def __init__(
         sink: TextIO,
         delimiter: str = "\t",
         lineterminator: str = "\n",
+        write_header: bool = True,
     ) -> None:
         """
-        Initialize a new `MetricWriter` and write the header row to `sink`.
+        Initialize a new `MetricWriter`.
+
+        By default the header row is written to `sink` immediately on construction. Pass
+        `write_header=False` to suppress it — e.g. when appending to a sink that already
+        contains a header.
 
         Args:
             metric_class: Metric class.
             sink: Writable text IO (e.g., file handle, StringIO) to write to.
             delimiter: The output file delimiter.
             lineterminator: The string used to terminate lines.
+            write_header: Whether to write the header row on construction.
         """
         self._metric_class = metric_class
         self._writer = DictWriter(
@@ -49,7 +163,8 @@ def __init__(
             delimiter=delimiter,
             lineterminator=lineterminator,
         )
-        self._writer.writeheader()
+        if write_header:
+            self._writer.writeheader()
 
     @classmethod
     @contextmanager
@@ -57,6 +172,7 @@ def open(
         cls,
         metric_class: type[T],
         path: Path | str,
+        mode: Literal["w", "a"] = "w",
         delimiter: str | None = None,
         lineterminator: str = "\n",
         encoding: str = "utf-8",
@@ -72,11 +188,25 @@ def open(
         Compression is selected automatically based on the output file extension: plaintext, gzip
         (`.gz`), bzip2 (`.bz2`), or xz (`.xz`).
 
-        The header is written on context entry; the file is closed on context exit.
+        With `mode="w"` (the default) the file is truncated and the header row is written (refused
+        with `FileExistsError` if the file already exists and `overwrite` is `False`). With
+        `mode="a"` the file is opened for appending: if it is missing or empty the header is
+        written first (append-or-create); if it already has content, its first row is inspected and
+        rows are appended without writing a header when that row either matches the expected header
+        or validates as a record of the metric class (a headerless data file). A `ValueError` is
+        raised only when the first row is neither. The first row is parsed with the same `delimiter`
+        and `encoding`, so a foreign file written with a different delimiter or a BOM will fail to
+        validate (`MetricReader.open` defaults to `utf-8-sig`, which strips a BOM; the writer does
+        not). `MetricWriter` always terminates rows with `lineterminator`, so appending to a foreign
+        file whose last line lacks a trailing newline would concatenate.
+
+        The file is opened lazily on context entry and closed on context exit.
 
         Args:
             metric_class: Metric class.
             path: Filesystem path to the output file.
+            mode: `"w"` to truncate and write, `"a"` to append. Append is non-destructive, so the
+                `overwrite` guard does not apply.
             delimiter: The output file delimiter. When `None` (the default), the delimiter is
                 inferred from the file extension: `.csv` → comma; `.tsv`, `.txt`, `.tab`, or
                 any extension ending in `metrics` → tab — ignoring any trailing compression
@@ -95,22 +225,35 @@ def open(
             IsADirectoryError: If `path` is a directory.
             PermissionError: If `path` exists but is not writable, or if `path` cannot be
                 created in its parent directory.
-            FileExistsError: If `path` already exists and `overwrite` is `False`.
-            ValueError: If `delimiter` is omitted and the delimiter cannot be inferred from
-                the file extension.
+            FileExistsError: If `path` already exists, `mode="w"`, and `overwrite` is `False`.
+            ValueError: If `delimiter` is omitted and cannot be inferred from the file
+                extension, or if `mode="a"` and the existing file's first row is neither the
+                expected header nor a valid record.
 
         Example:
             ```python
             with MetricWriter.open(AlignmentMetric, "metrics.txt") as writer:
                 writer.writeall(metrics)
+
+            with MetricWriter.open(AlignmentMetric, "metrics.txt", mode="a") as writer:
+                writer.writeall(more_metrics)
             ```
         """
-        if (error := path_write_error(path, overwrite=overwrite)) is not None:
+        if (error := path_write_error(path, overwrite=overwrite or mode == "a")) is not None:
             raise error
         if delimiter is None:
             delimiter = infer_delimiter(path)
-        with xopen(path, mode="wt", encoding=encoding) as handle:
-            yield cls(metric_class, handle, delimiter, lineterminator)
+
+        xmode: Literal["wt", "at"]
+        if mode == "a":
+            xmode = "at"
+            write_header = _append_needs_header(metric_class, path, delimiter, encoding)
+        else:
+            xmode = "wt"
+            write_header = True
+
+        with xopen(path, mode=xmode, encoding=encoding) as handle:
+            yield cls(metric_class, handle, delimiter, lineterminator, write_header=write_header)
 
     def write(self, metric: T) -> None:
         """
diff --git a/tests/test_metric_writer.py b/tests/test_metric_writer.py
index 2cf4610..e79e315 100644
--- a/tests/test_metric_writer.py
+++ b/tests/test_metric_writer.py
@@ -12,6 +12,7 @@
 from pytest_mock import MockerFixture
 
 from fgmetric import Metric
+from fgmetric import MetricReader
 from fgmetric import MetricWriter
 from fgmetric import metric_writer
 
@@ -261,3 +262,133 @@ def test_writer_open_overwrites_existing_file_when_overwrite_true(tmp_path: Path
     with MetricWriter.open(FakeMetric, p, overwrite=True) as writer:
         writer.write(FakeMetric(foo="abc", bar=1))
     assert p.read_text() == "foo\tbar\nabc\t1\n"
+
+
+def test_writer_constructor_can_skip_header() -> None:
+    """A writer constructed with write_header=False writes no header row."""
+    sink = StringIO()
+    writer = MetricWriter(FakeMetric, sink, write_header=False)
+    writer.write(FakeMetric(foo="abc", bar=1))
+    assert sink.getvalue() == "abc\t1\n"
+
+
+def test_writer_append_skips_header_on_existing_file(tmp_path: Path) -> None:
+    """Appending to a file that already has a matching header does not rewrite it."""
+    p = tmp_path / "out.tsv"
+    with MetricWriter.open(FakeMetric, p, mode="w") as writer:
+        writer.write(FakeMetric(foo="a", bar=1))
+    with MetricWriter.open(FakeMetric, p, mode="a") as writer:
+        writer.write(FakeMetric(foo="b", bar=2))
+    assert p.read_text() == "foo\tbar\na\t1\nb\t2\n"
+
+
+def test_writer_append_writes_header_when_file_missing(tmp_path: Path) -> None:
+    """Append-or-create: appending to a missing file writes the header first."""
+    p = tmp_path / "out.tsv"  # does not exist
+    with MetricWriter.open(FakeMetric, p, mode="a") as writer:
+        writer.write(FakeMetric(foo="a", bar=1))
+    assert p.read_text() == "foo\tbar\na\t1\n"
+
+
+def test_writer_append_writes_header_when_file_empty(tmp_path: Path) -> None:
+    """Appending to an existing empty file writes the header first."""
+    p = tmp_path / "out.tsv"
+    p.touch()
+    with MetricWriter.open(FakeMetric, p, mode="a") as writer:
+        writer.write(FakeMetric(foo="a", bar=1))
+    assert p.read_text() == "foo\tbar\na\t1\n"
+
+
+def test_writer_append_accepts_matching_header(tmp_path: Path) -> None:
+    """Appending to a file whose header matches the metric fields succeeds."""
+    p = tmp_path / "out.tsv"
+    p.write_text("foo\tbar\n")
+    with MetricWriter.open(FakeMetric, p, mode="a") as writer:
+        writer.write(FakeMetric(foo="a", bar=1))
+    assert p.read_text() == "foo\tbar\na\t1\n"
+
+
+def test_writer_append_accepts_headerless_data_file(tmp_path: Path) -> None:
+    """Appending to a headerless file whose first row validates appends without a header."""
+    p = tmp_path / "out.tsv"
+    p.write_text("abc\t1\ndef\t2\n")  # data rows, no header
+    with MetricWriter.open(FakeMetric, p, mode="a") as writer:
+        writer.write(FakeMetric(foo="x", bar=9))
+    # The file is recognized as headerless data, so no header is injected mid-file.
+    assert p.read_text() == "abc\t1\ndef\t2\nx\t9\n"
+
+
+def test_writer_append_to_headerless_file_reads_back_with_fieldnames(tmp_path: Path) -> None:
+    """A headerless file stays headerless after appending and reads back with `fieldnames`."""
+    p = tmp_path / "out.tsv"
+    p.write_text("abc\t1\n")
+    with MetricWriter.open(FakeMetric, p, mode="a") as writer:
+        writer.write(FakeMetric(foo="x", bar=9))
+    with MetricReader.open(FakeMetric, p, fieldnames=["foo", "bar"]) as reader_:
+        assert list(reader_) == [FakeMetric(foo="abc", bar=1), FakeMetric(foo="x", bar=9)]
+
+
+def test_writer_append_raises_when_first_row_neither_header_nor_record(tmp_path: Path) -> None:
+    """Appending raises when the first row is neither the header nor a valid record."""
+    p = tmp_path / "out.tsv"
+    # `foo` accepts any string, but `bar` cannot parse as an int, so this is not a valid record;
+    # nor does it match the `foo\tbar` header.
+    p.write_text("abc\tnot_an_int\n")
+    with pytest.raises(ValueError, match="neither"):
+        with MetricWriter.open(FakeMetric, p, mode="a"):
+            pass
+
+
+def test_writer_append_raises_when_first_row_has_wrong_column_count(tmp_path: Path) -> None:
+    """A first row with the wrong number of columns is neither header nor record, so it raises."""
+    p = tmp_path / "out.tsv"
+    p.write_text("only_one_column\n")
+    with pytest.raises(ValueError, match="neither"):
+        with MetricWriter.open(FakeMetric, p, mode="a"):
+            pass
+
+
+def test_writer_open_append_does_not_touch_file_until_enter(tmp_path: Path) -> None:
+    """open(mode="a") must not read or write the file until the context is entered."""
+    p = tmp_path / "out.tsv"
+    p.write_text("foo\tbar\n")
+    MetricWriter.open(FakeMetric, p, mode="a")
+    assert p.read_text() == "foo\tbar\n"
+
+
+@pytest.mark.parametrize("suffix", ["", ".gz", ".bz2", ".xz"])
+def test_writer_append_round_trips_across_formats(tmp_path: Path, suffix: str) -> None:
+    """Write then append then read back yields all rows, header-once, for every format."""
+    p = tmp_path / f"out.tsv{suffix}"
+    with MetricWriter.open(FakeMetric, p, mode="w") as writer:
+        writer.write(FakeMetric(foo="a", bar=1))
+    with MetricWriter.open(FakeMetric, p, mode="a") as writer:
+        writer.write(FakeMetric(foo="b", bar=2))
+
+    with MetricReader.open(FakeMetric, p) as reader_:
+        got = list(reader_)
+
+    assert got == [FakeMetric(foo="a", bar=1), FakeMetric(foo="b", bar=2)]
+
+
+@pytest.mark.parametrize("suffix", ["", ".gz", ".bz2", ".xz"])
+def test_writer_append_to_empty_file_writes_header_across_formats(
+    tmp_path: Path, suffix: str
+) -> None:
+    """Append-or-create to an existing empty file writes the header first, for every format."""
+    p = tmp_path / f"out.tsv{suffix}"
+    p.touch()  # 0-byte file
+    with MetricWriter.open(FakeMetric, p, mode="a") as writer:
+        writer.write(FakeMetric(foo="a", bar=1))
+
+    with MetricReader.open(FakeMetric, p) as reader_:
+        assert list(reader_) == [FakeMetric(foo="a", bar=1)]
+
+
+def test_writer_append_concatenates_when_last_line_lacks_newline(tmp_path: Path) -> None:
+    """Appending after a newline-less last line concatenates the new row, as documented."""
+    p = tmp_path / "out.tsv"
+    p.write_text("foo\tbar\na\t1")  # header + one row, no trailing newline
+    with MetricWriter.open(FakeMetric, p, mode="a") as writer:
+        writer.write(FakeMetric(foo="b", bar=2))
+    assert p.read_text() == "foo\tbar\na\t1b\t2\n"