diff --git a/src/picopip.py b/src/picopip.py
index c5fce2d..0babb20 100644
--- a/src/picopip.py
+++ b/src/picopip.py
@@ -17,6 +17,7 @@
 
 import itertools
 import logging
+import os
 import re
 import site
 from importlib.metadata import PathDistribution
@@ -62,11 +63,8 @@ def get_site_package_paths(
             continue
 
     if include_system_packages:
-        # Append system packages at the end, so that venv site-packages take precedence
-        for system_path in _find_system_packages(venv_path):
-            if system_path not in seen:
-                scan_paths.append(system_path)
-                seen.add(system_path)
+        _extend_unique(scan_paths, seen, _find_system_packages(venv_path))
+        _extend_unique(scan_paths, seen, _find_pythonpath_packages())
 
     return scan_paths
 
@@ -125,6 +123,30 @@ def get_package_version_from_env(venv_path: str, package_name: str) -> Optional[
     return None
 
 
+def _extend_unique(scan_paths: List[Path], seen: set, new_paths: List[Path]) -> None:
+    """Append paths to scan_paths that are not already in seen."""
+    for path in new_paths:
+        if path not in seen:
+            scan_paths.append(path)
+            seen.add(path)
+
+
+def _find_pythonpath_packages() -> List[Path]:
+    """Return existing directories listed in the PYTHONPATH environment variable.
+
+    PYTHONPATH is read from the process running picopip, which may differ from
+    the interpreter that will import from the venv. Results are only meaningful
+    when the two processes share the same PYTHONPATH value.
+    """
+    scan_paths = []
+    for entry in os.environ.get("PYTHONPATH", "").split(os.pathsep):
+        if entry:
+            pp = Path(entry).resolve()
+            if pp.exists() and pp.is_dir():
+                scan_paths.append(pp)
+    return scan_paths
+
+
 def _find_system_packages(venv_path: str) -> List[Path]:
     """Return scan paths for system packages if enabled in the venv."""
     scan_paths = []
diff --git a/tests/test_packages_discovery.py b/tests/test_packages_discovery.py
index 77f8781..b7826c8 100644
--- a/tests/test_packages_discovery.py
+++ b/tests/test_packages_discovery.py
@@ -1,3 +1,4 @@
+import os
 import subprocess
 import tempfile
 import venv
@@ -208,6 +209,92 @@ def test_get_packages_from_env_ignore_system_packages(tmp_path, monkeypatch):
     assert ("system-pkg", "0.1.0") not in pkgs
 
 
+def test_get_site_package_paths_includes_pythonpath(fake_venv, tmp_path, monkeypatch):
+    """PYTHONPATH directories should be included in scan paths."""
+    venv, _site = fake_venv
+    pythonpath_dir = tmp_path / "pythonpath_packages"
+    pythonpath_dir.mkdir()
+    monkeypatch.setenv("PYTHONPATH", str(pythonpath_dir))
+    paths = get_site_package_paths(str(venv))
+    assert pythonpath_dir.resolve() in [p.resolve() for p in paths]
+
+
+def test_get_site_package_paths_excludes_pythonpath_without_system(
+    fake_venv, tmp_path, monkeypatch
+):
+    """PYTHONPATH should be excluded when include_system_packages is False."""
+    venv, _site = fake_venv
+    pythonpath_dir = tmp_path / "pythonpath_packages"
+    pythonpath_dir.mkdir()
+    monkeypatch.setenv("PYTHONPATH", str(pythonpath_dir))
+    paths = get_site_package_paths(str(venv), include_system_packages=False)
+    assert pythonpath_dir.resolve() not in [p.resolve() for p in paths]
+
+
+def test_get_site_package_paths_pythonpath_empty(fake_venv, monkeypatch):
+    """Empty PYTHONPATH should not cause errors."""
+    venv, _site = fake_venv
+    monkeypatch.setenv("PYTHONPATH", "")
+    paths = get_site_package_paths(str(venv))
+    assert len(paths) >= 1
+
+
+def test_get_site_package_paths_pythonpath_unset(fake_venv, monkeypatch):
+    """Unset PYTHONPATH should not cause errors."""
+    venv, _site = fake_venv
+    monkeypatch.delenv("PYTHONPATH", raising=False)
+    paths = get_site_package_paths(str(venv))
+    assert len(paths) >= 1
+
+
+def test_get_site_package_paths_pythonpath_nonexistent(fake_venv, monkeypatch):
+    """Nonexistent PYTHONPATH directories should be silently ignored."""
+    venv, _site = fake_venv
+    monkeypatch.setenv("PYTHONPATH", "/nonexistent/path/that/does/not/exist")
+    paths = get_site_package_paths(str(venv))
+    assert not any(str(p) == "/nonexistent/path/that/does/not/exist" for p in paths)
+
+
+def test_get_site_package_paths_pythonpath_multiple(fake_venv, tmp_path, monkeypatch):
+    """Multiple PYTHONPATH entries should all be included."""
+    venv, _site = fake_venv
+    dir_a = tmp_path / "path_a"
+    dir_b = tmp_path / "path_b"
+    dir_a.mkdir()
+    dir_b.mkdir()
+    monkeypatch.setenv("PYTHONPATH", os.pathsep.join([str(dir_a), str(dir_b)]))
+    paths = get_site_package_paths(str(venv))
+    resolved = [p.resolve() for p in paths]
+    assert dir_a.resolve() in resolved
+    assert dir_b.resolve() in resolved
+
+
+def test_get_packages_from_env_finds_pythonpath_packages(
+    fake_venv, tmp_path, monkeypatch
+):
+    """Packages in PYTHONPATH directories should be discovered."""
+    venv, _site = fake_venv
+    pythonpath_dir = tmp_path / "pythonpath_packages"
+    pythonpath_dir.mkdir()
+    make_dist_info(pythonpath_dir, "external-pkg", "2.0.0")
+    monkeypatch.setenv("PYTHONPATH", str(pythonpath_dir))
+    pkgs = get_packages_from_env(str(venv))
+    assert ("external-pkg", "2.0.0") in pkgs
+
+
+def test_get_packages_from_env_ignores_pythonpath_with_ignore_system(
+    fake_venv, tmp_path, monkeypatch
+):
+    """PYTHONPATH packages should be excluded when ignore_system_packages is True."""
+    venv, _site = fake_venv
+    pythonpath_dir = tmp_path / "pythonpath_packages"
+    pythonpath_dir.mkdir()
+    make_dist_info(pythonpath_dir, "external-pkg", "2.0.0")
+    monkeypatch.setenv("PYTHONPATH", str(pythonpath_dir))
+    pkgs = get_packages_from_env(str(venv), ignore_system_packages=True)
+    assert ("external-pkg", "2.0.0") not in pkgs
+
+
 def test_e2e_readme_example():
     with tempfile.TemporaryDirectory() as tmpdir:
         venv.create(tmpdir, with_pip=True)