diff --git a/datasets/modis/modis.py b/datasets/modis/modis.py
index 56735b5f4..2d9cf501e 100644
--- a/datasets/modis/modis.py
+++ b/datasets/modis/modis.py
@@ -4,8 +4,13 @@
 from typing import List, Union
 
 import pystac
+import rasterio
+import shapely.geometry
 import stactools.modis.cog
 import stactools.modis.stac
+import stactools.modis.utils
+from pystac.extensions.projection import ProjectionExtension
+from rasterio.enums import WktVersion
 from stactools.modis.file import File
 
 from pctasks.core.models.task import WaitTaskResult
@@ -58,6 +63,32 @@ def add_platform_field(item, href, logger):
     return item
 
 
+def add_projection(item: pystac.Item, hdf_path: str) -> pystac.Item:
+    """Add the projection extension, read from the item's local HDF file.
+
+    stactools-modis only sets projection when its builder reads the COGs, and
+    this collection attaches COGs with ``add_cogs`` / ``add_cog_assets``, which
+    don't. Every asset of a MODIS item shares one sinusoidal tile grid, so the
+    HDF's first subdataset gives the projection for the whole item.
+    """
+    subdatasets = stactools.modis.utils.subdatasets(hdf_path)
+    if not subdatasets:
+        raise ValueError(f"No subdatasets found in HDF file: {hdf_path}")
+    with rasterio.open(subdatasets[0]) as dataset:
+        crs = dataset.crs
+        bounds = dataset.bounds
+        transform = list(dataset.transform)[0:6]
+        shape = list(dataset.shape)
+
+    projection = ProjectionExtension.ext(item, add_if_missing=True)
+    projection.epsg = crs.to_epsg()
+    projection.wkt2 = crs.to_wkt(version=WktVersion.WKT2_2019)
+    projection.geometry = shapely.geometry.mapping(shapely.geometry.box(*bounds))
+    projection.transform = transform
+    projection.shape = shape
+    return item
+
+
 class MODISCollection(Collection):
     @classmethod
     def create_item(
@@ -86,6 +117,7 @@ def create_item(
 
             logger.debug("Creating item")
             item = stactools.modis.stac.create_item(file.hdf_href)
+            add_projection(item, file.hdf_href)
 
             if create_cogs:
                 logger.debug(f"Adding COGS to item {item}")
diff --git a/datasets/modis/tests/test_projection.py b/datasets/modis/tests/test_projection.py
new file mode 100644
index 000000000..d1f8add1b
--- /dev/null
+++ b/datasets/modis/tests/test_projection.py
@@ -0,0 +1,63 @@
+from datetime import datetime
+from pathlib import Path
+
+import numpy as np
+import pystac
+import pytest
+import rasterio
+from affine import Affine
+from rasterio.crs import CRS
+
+import modis
+
+# MODIS sinusoidal grid: tile h23v07 at 500 m.
+SINUSOIDAL = CRS.from_proj4("+proj=sinu +lon_0=0 +x_0=0 +y_0=0 +R=6371007.181 +units=m +no_defs")
+PIXEL = 463.3127165275
+ORIGIN = (5559752.598333, 2223901.039333)
+SHAPE = (2400, 2400)
+
+
+@pytest.fixture
+def subdataset(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> str:
+    """A GeoTIFF on the MODIS grid standing in for the HDF's first subdataset."""
+    path = str(tmp_path / "subdataset.tif")
+    with rasterio.open(
+        path,
+        "w",
+        driver="GTiff",
+        width=SHAPE[1],
+        height=SHAPE[0],
+        count=1,
+        dtype="uint8",
+        crs=SINUSOIDAL,
+        transform=Affine(PIXEL, 0.0, ORIGIN[0], 0.0, -PIXEL, ORIGIN[1]),
+    ) as dataset:
+        dataset.write(np.zeros((1, *SHAPE), dtype="uint8"))
+    monkeypatch.setattr(modis.stactools.modis.utils, "subdatasets", lambda href: [path])
+    return path
+
+
+def test_add_projection_reads_the_hdf_grid(subdataset: str) -> None:
+    item = pystac.Item("MOD10A1.A2000057.h23v07.061.2020037182526", None, None, datetime(2000, 2, 26), {})
+
+    modis.add_projection(item, "MOD10A1.A2000057.h23v07.061.2020037182526.hdf")
+
+    projection = pystac.extensions.projection.ProjectionExtension.ext(item)
+    assert projection.has_extension(item)
+    assert projection.epsg is None
+    assert projection.wkt2.startswith("PROJCRS[")
+    assert CRS.from_wkt(projection.wkt2) == SINUSOIDAL
+    assert projection.shape == list(SHAPE)
+    assert projection.transform == pytest.approx([PIXEL, 0.0, ORIGIN[0], 0.0, -PIXEL, ORIGIN[1]])
+    min_x, max_y = ORIGIN
+    max_x, min_y = min_x + PIXEL * SHAPE[1], max_y - PIXEL * SHAPE[0]
+    ring = [coordinate for point in projection.geometry["coordinates"][0] for coordinate in point]
+    assert ring == pytest.approx([max_x, min_y, max_x, max_y, min_x, max_y, min_x, min_y, max_x, min_y])
+
+
+def test_add_projection_requires_subdatasets(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setattr(modis.stactools.modis.utils, "subdatasets", lambda href: [])
+    item = pystac.Item("item", None, None, datetime(2000, 2, 26), {})
+
+    with pytest.raises(ValueError, match="No subdatasets"):
+        modis.add_projection(item, "empty.hdf")