diff --git a/pygmt/datasets/earth_mask.py b/pygmt/datasets/earth_mask.py
index 5b10357e5e8..371e0764dda 100644
--- a/pygmt/datasets/earth_mask.py
+++ b/pygmt/datasets/earth_mask.py
@@ -89,4 +89,7 @@ def load_earth_mask(resolution="01d", region=None, registration=None):
         region=region,
         registration=registration,
     )
-    return grid.astype("int8")
+    # `return grid.astype("int8")` doesn't work because grid encoding is lost.
+    # See https://github.com/GenericMappingTools/pygmt/issues/2629.
+    grid.data = grid.data.astype("int8")
+    return grid
diff --git a/pygmt/tests/test_datasets_earth_mask.py b/pygmt/tests/test_datasets_earth_mask.py
index 27ded0e92a7..0f55b76aa10 100644
--- a/pygmt/tests/test_datasets_earth_mask.py
+++ b/pygmt/tests/test_datasets_earth_mask.py
@@ -37,6 +37,7 @@ def test_earth_mask_01d():
     assert data.attrs["horizontal_datum"] == "WGS84"
     assert data.shape == (181, 361)
     assert data.gmt.registration == 0
+    assert data.gmt.gtype == 1
     assert data.dtype == "int8"
     npt.assert_allclose(data.lat, np.arange(-90, 91, 1))
     npt.assert_allclose(data.lon, np.arange(-180, 181, 1))
@@ -52,6 +53,8 @@ def test_earth_mask_01d_with_region():
     data = load_earth_mask(resolution="01d", region=[-7, 4, 13, 19])
     assert data.shape == (7, 12)
     assert data.gmt.registration == 0
+    assert data.gmt.gtype == 1
+    assert data.dtype == "int8"
     npt.assert_allclose(data.lat, np.arange(13, 20, 1))
     npt.assert_allclose(data.lon, np.arange(-7, 5, 1))
     npt.assert_allclose(data[1, 5], 1)