diff --git a/changelog/13091.doc.rst b/changelog/13091.doc.rst
new file mode 100644
index 00000000000..2bfd8c1d523
--- /dev/null
+++ b/changelog/13091.doc.rst
@@ -0,0 +1 @@
+Added Sphinx-compatible docstrings and inline comments for basetemp function in `pytest/src/_pytest/main.py`
diff --git a/src/_pytest/main.py b/src/_pytest/main.py
index dac084b553a..5f9683eb19a 100644
--- a/src/_pytest/main.py
+++ b/src/_pytest/main.py
@@ -249,6 +249,18 @@ def pytest_addoption(parser: Parser) -> None:
 
 
 def validate_basetemp(path: str) -> str:
+    """
+    Validate the provided `basetemp` path to ensure it is not empty,
+    the current working directory (cwd), or any ancestor of the cwd.
+    Returns the validated basetemp path.
+    Raises argparse.ArgumentTypeError in 3 cases:
+            1) if the path is empty;
+            2) if it is the cwd;
+            3) if it is an ancestor of the cwd.
+
+
+    param: path (str): The basetemp path.
+    """
     # GH 7119
     msg = "basetemp must not be empty, the current working directory or any parent directory of it"