diff --git a/src/_pytest/config/findpaths.py b/src/_pytest/config/findpaths.py
index 3f08c8121a9..baa322e15ee 100644
--- a/src/_pytest/config/findpaths.py
+++ b/src/_pytest/config/findpaths.py
@@ -5,6 +5,7 @@
 import os
 from pathlib import Path
 import sys
+import tempfile
 from typing import TypeAlias
 
 import iniconfig
@@ -184,6 +185,28 @@ def get_dir_from_path(path: Path) -> Path:
 CFG_PYTEST_SECTION = "[pytest] section in {filename} files is no longer supported, change to [tool:pytest] instead."
 
 
+def _is_system_temp_or_parent(path: Path) -> bool:
+    """Check if path is the system temp directory.
+
+    This prevents pytest from treating /tmp as a valid rootdir when
+    a /tmp/setup.py file exists on the system.
+
+    Args:
+        path: The path to check
+
+    Returns:
+        True if path is the system temp directory, False otherwise
+    """
+    try:
+        system_temp = Path(tempfile.gettempdir()).resolve()
+        path = path.resolve()
+        # Skip exact matches with the system temp directory
+        return path == system_temp
+    except (OSError, RuntimeError):
+        # If we can't resolve paths, play it safe and don't skip
+        return False
+
+
 def determine_setup(
     *,
     inifile: str | None,
@@ -220,6 +243,9 @@ def determine_setup(
         )
         if rootdir is None and rootdir_cmd_arg is None:
             for possible_rootdir in (ancestor, *ancestor.parents):
+                # Skip system temp directories to avoid false positives (issue #13822)
+                if _is_system_temp_or_parent(possible_rootdir):
+                    continue
                 if (possible_rootdir / "setup.py").is_file():
                     rootdir = possible_rootdir
                     break
diff --git a/testing/test_config.py b/testing/test_config.py
index 9efac8739b7..4241de44fe4 100644
--- a/testing/test_config.py
+++ b/testing/test_config.py
@@ -1830,6 +1830,42 @@ def test_with_config_also_in_parent_directory(
         assert rootpath == tmp_path / "myproject"
         assert inipath == tmp_path / "myproject" / "setup.cfg"
 
+    def test_tmp_setup_py_does_not_affect_rootdir(
+        self, tmp_path: Path, monkeypatch: MonkeyPatch
+    ) -> None:
+        """Regression test for issue #13822.
+
+        Ensure that setup.py in the system temp directory doesn't
+        affect rootdir detection.
+        """
+        # Create a fake project structure
+        project = tmp_path / "my_project"
+        project.mkdir()
+        test_dir = project / "tests"
+        test_dir.mkdir()
+
+        # Create project's own setup.py
+        (project / "setup.py").write_text("# project setup", "utf-8")
+
+        # Monkeypatch tempfile.gettempdir() to simulate /tmp/setup.py existing
+        fake_temp = tmp_path / "fake_tmp"
+        fake_temp.mkdir()
+        (fake_temp / "setup.py").write_text("# tmp setup", "utf-8")
+
+        monkeypatch.setattr("tempfile.gettempdir", lambda: str(fake_temp))
+        monkeypatch.chdir(test_dir)
+
+        rootpath, inipath, *_ = determine_setup(
+            inifile=None,
+            args=[str(test_dir)],
+            rootdir_cmd_arg=None,
+            invocation_dir=test_dir,
+        )
+
+        # Rootpath should be project, not fake temp
+        assert rootpath == project
+        assert rootpath != fake_temp
+
 
 class TestOverrideIniArgs:
     @pytest.mark.parametrize("name", "setup.cfg tox.ini pytest.ini".split())