diff --git a/src/pip/_internal/build_env.py b/src/pip/_internal/build_env.py
index 1a42a9d41..7639dabca 100644
--- a/src/pip/_internal/build_env.py
+++ b/src/pip/_internal/build_env.py
@@ -468,15 +468,19 @@ class BuildEnvironment:
                     """
                 import os, site, sys
 
-                # First, drop system-sites related paths.
+                # First, discover all system-sites related paths.
                 original_sys_path = sys.path[:]
+                # Clear sys.path so addsitedir() will add system site paths and paths
+                # added by contained .pth files to sys.path reliably. This is necessary
+                # since Python 3.15, which notably no longer re-executes .pth files for
+                # known paths.
+                sys.path = []
                 known_paths = set()
                 for path in {system_sites!r}:
                     site.addsitedir(path, known_paths=known_paths)
-                system_paths = set(
-                    os.path.normcase(path)
-                    for path in sys.path[len(original_sys_path):]
-                )
+                system_paths = set(os.path.normcase(path) for path in sys.path)
+
+                # Drop discovered system-sites related paths.
                 original_sys_path = [
                     path for path in original_sys_path
                     if os.path.normcase(path) not in system_paths
diff --git a/tests/lib/venv.py b/tests/lib/venv.py
index 67b01d9f3..4e86b92b3 100644
--- a/tests/lib/venv.py
+++ b/tests/lib/venv.py
@@ -174,11 +174,13 @@ class VirtualEnvironment:
                     site.ENABLE_USER_SITE = {self._user_site_packages}
                     # First, drop system-sites related paths.
                     original_sys_path = sys.path[:]
+                    # To discover system-sites related paths, clear sys.path
+                    # and build a new one with only system paths.
+                    sys.path = []
                     known_paths = set()
                     for path in site.getsitepackages():
                         site.addsitedir(path, known_paths=known_paths)
-                    system_paths = sys.path[len(original_sys_path):]
-                    for path in system_paths:
+                    for path in sys.path:
                         if path in original_sys_path:
                             original_sys_path.remove(path)
                     sys.path = original_sys_path