diff --git a/changelog/4287.bugfix.rst b/changelog/4287.bugfix.rst
new file mode 100644
index 000000000..5ba5fbb7a
--- /dev/null
+++ b/changelog/4287.bugfix.rst
@@ -0,0 +1 @@
+Fix nested usage of debugging plugin (pdb), e.g. with pytester's ``testdir.runpytest``.
diff --git a/src/_pytest/debugging.py b/src/_pytest/debugging.py
index 5a9729d5b..0baa84afa 100644
--- a/src/_pytest/debugging.py
+++ b/src/_pytest/debugging.py
@@ -47,17 +47,24 @@ def pytest_configure(config):
     if config.getvalue("usepdb"):
         config.pluginmanager.register(PdbInvoke(), "pdbinvoke")
 
-    old = (pdb.set_trace, pytestPDB._pluginmanager)
-
-    def fin():
-        pdb.set_trace, pytestPDB._pluginmanager = old
-        pytestPDB._config = None
-        pytestPDB._pdb_cls = pdb.Pdb
-
+    pytestPDB._saved.append(
+        (pdb.set_trace, pytestPDB._pluginmanager, pytestPDB._config, pytestPDB._pdb_cls)
+    )
     pdb.set_trace = pytestPDB.set_trace
     pytestPDB._pluginmanager = config.pluginmanager
     pytestPDB._config = config
     pytestPDB._pdb_cls = pdb_cls
+
+    # NOTE: not using pytest_unconfigure, since it might get called although
+    #       pytest_configure was not (if another plugin raises UsageError).
+    def fin():
+        (
+            pdb.set_trace,
+            pytestPDB._pluginmanager,
+            pytestPDB._config,
+            pytestPDB._pdb_cls,
+        ) = pytestPDB._saved.pop()
+
     config._cleanup.append(fin)
 
 
@@ -67,6 +74,7 @@ class pytestPDB(object):
     _pluginmanager = None
     _config = None
     _pdb_cls = pdb.Pdb
+    _saved = []
 
     @classmethod
     def set_trace(cls, set_break=True):
diff --git a/testing/test_pdb.py b/testing/test_pdb.py
index 56fe5fc7a..ec1af5b56 100644
--- a/testing/test_pdb.py
+++ b/testing/test_pdb.py
@@ -784,3 +784,30 @@ class TestTraceOption:
         assert "1 passed" in rest
         assert "reading from stdin while output" not in rest
         TestPDB.flush(child)
+
+
+def test_trace_after_runpytest(testdir):
+    """Test that debugging's pytest_configure is re-entrant."""
+    p1 = testdir.makepyfile(
+        """
+        from _pytest.debugging import pytestPDB
+
+        def test_outer(testdir):
+            from _pytest.debugging import pytestPDB
+
+            assert len(pytestPDB._saved) == 1
+
+            testdir.runpytest("-k test_inner")
+
+            __import__('pdb').set_trace()
+
+        def test_inner(testdir):
+            assert len(pytestPDB._saved) == 2
+    """
+    )
+    child = testdir.spawn_pytest("-p pytester %s -k test_outer" % p1)
+    child.expect(r"\(Pdb")
+    child.sendline("c")
+    rest = child.read().decode("utf8")
+    TestPDB.flush(child)
+    assert child.exitstatus == 0, rest