diff --git a/tests/test_pkg.py b/tests/test_pkg.py
index c47516a5796..7455e97cbdf 100644
--- a/tests/test_pkg.py
+++ b/tests/test_pkg.py
@@ -1,35 +1,107 @@
+"""Check the content of source code and test build source distribution.
+
+Binary distribution (wheel) is checked in test workflow.
+"""
+
 from __future__ import annotations
 
-import os
+import subprocess
+import tarfile
 from glob import glob
+from pathlib import Path
+from typing import TYPE_CHECKING
+
+from monty.tempfile import ScratchDir
+
+if TYPE_CHECKING:
+    from pymatgen.util.typing import PathLike
+
+PROJECT_ROOT = Path(__file__).parent.parent
+NAMESPACE_PKGS = {"analysis", "ext", "io"}  # TODO: double check
+
+
+def test_source_code():
+    """Check the source code in the working directory."""
+    src_txt_path = PROJECT_ROOT / "src/pymatgen.egg-info/SOURCES.txt"
+
+    _check_src_txt_is_complete(PROJECT_ROOT, src_txt_path)
+
+    # Check existence of `py.typed` file in each sub-package
+    _check_py_typed_files(PROJECT_ROOT / "src/pymatgen")
+
+
+def test_build_source_distribution():
+    """Test build the source distribution (sdist), also check `py.typed` files."""
+    with ScratchDir("."):
+        # Build the source distribution
+        subprocess.run(["python", "-m", "pip", "install", "--upgrade", "build"], check=True)
+        subprocess.run(["python", "-m", "build", "--sdist", PROJECT_ROOT, "--outdir", ".", "-C--quiet"], check=True)
 
-import pytest
+        # Decompress sdist
+        sdist_file = next(Path(".").glob("*.tar.gz"))
+        sdist_dir = Path(sdist_file.name.removesuffix(".tar.gz"))
+        with tarfile.open(sdist_file, "r:gz") as tar:
+            # TODO: remove attr check after only 3.12+
+            if hasattr(tarfile, "data_filter"):
+                tar.extractall("", filter="data")
+            else:
+                tar.extractall("")  # noqa: S202
 
-SRC_TXT_PATH = "src/pymatgen.egg-info/SOURCES.txt"
+        # Check existence of `py.typed` file in each sub-package
+        _check_py_typed_files(sdist_dir / "src/pymatgen")
 
 
-@pytest.mark.skipif(
-    not os.path.isfile(SRC_TXT_PATH),
-    reason=f"{SRC_TXT_PATH=} not found. Run `pip install .` to create",
-)
-def test_egg_sources_txt_is_complete():
-    """Check that all source and data files in pymatgen/ are listed in pymatgen.egg-info/SOURCES.txt."""
+def _check_src_txt_is_complete(project_root: PathLike, src_txt_path: PathLike) -> None:
+    """Check that all source code and data files are listed in given SOURCES.txt.
 
-    with open(SRC_TXT_PATH, encoding="utf-8") as file:
-        sources = file.read()
+    Args:
+        project_root (PathLike): Path to the directory containing "src/pymatgen/".
+        src_txt_path (PathLike): Path to the "SOURCES.txt" file.
+    """
+    project_root = Path(project_root)
+    src_txt_path = Path(src_txt_path)
 
-    # check that all files listed in SOURCES.txt exist
+    assert project_root.is_dir(), f"{project_root} is not a directory"
+    assert src_txt_path.is_file(), f"{src_txt_path} doesn't exist"
+
+    sources = src_txt_path.read_text(encoding="utf-8")
+
+    # Check that all files listed in "SOURCES.txt" exist
     for src_file in sources.splitlines():
-        assert os.path.isfile(src_file), f"{src_file!r} does not exist!"
-
-    # check that all files in pymatgen/ are listed in SOURCES.txt
-    for ext in ("py", "json*", "yaml", "csv"):
-        for filepath in glob(f"pymatgen/**/*.{ext}", recursive=True):
-            unix_path = filepath.replace("\\", "/")
-            if unix_path.endswith("dao.py"):
-                continue
+        assert (project_root / src_file).is_file(), f"{src_file!r} does not exist!"
+
+    # Check that all files in src/pymatgen/ are listed in SOURCES.txt
+    for ext in ("py", "json", "json.*", "yaml", "csv"):
+        for filepath in glob(f"{project_root}/src/pymatgen/**/*.{ext}", recursive=True):
+            unix_path = Path(filepath).relative_to(project_root).as_posix()
+
             if unix_path not in sources:
-                raise ValueError(
-                    f"{unix_path} not found in {SRC_TXT_PATH}. check setup.py package_data for "
-                    "outdated inclusion rules."
-                )
+                raise ValueError(f"{unix_path} not found in {src_txt_path}, check package data config")
+
+
+def _check_py_typed_files(pkg_root: PathLike) -> None:
+    """
+    Ensure all sub-packages contain a `py.typed` file.
+
+    Args:
+        pkg_root (PathLike): Path to the namespace package's root directory.
+
+    Raises:
+        FileNotFoundError: If any sub-package is missing the `py.typed` file.
+    """
+    if not (pkg_root := Path(pkg_root)).is_dir():
+        raise NotADirectoryError(f"Provided path {pkg_root} is not a valid directory.")
+
+    # Iterate through all directories under the namespace package
+    for sub_pkg in pkg_root.glob("*/"):
+        if sub_pkg.name.startswith(".") or sub_pkg.name.startswith("__"):
+            continue
+
+        # Check for __init__.py to ensure it's not a namespace package
+        sub_pkg_path = Path(sub_pkg)
+        if (sub_pkg_path / "__init__.py").exists():
+            if not (sub_pkg_path / "py.typed").exists():
+                raise FileNotFoundError(f"Missing py.typed in sub-package: {sub_pkg_path}")
+
+        elif sub_pkg_path.name not in NAMESPACE_PKGS:
+            raise ValueError(f"Unexpected namespace package {sub_pkg_path.name}")