diff --git a/changelog/4855.feature.rst b/changelog/4855.feature.rst
new file mode 100644
index 000000000..274d3991f
--- /dev/null
+++ b/changelog/4855.feature.rst
@@ -0,0 +1,4 @@
+The ``--pdbcls`` option handles classes via module attributes now (e.g.
+``pdb:pdb.Pdb`` with `pdb++`_), and its validation was improved.
+
+.. _pdb++: https://pypi.org/project/pdbpp/
diff --git a/src/_pytest/debugging.py b/src/_pytest/debugging.py
index 6b401aa0b..271a590a1 100644
--- a/src/_pytest/debugging.py
+++ b/src/_pytest/debugging.py
@@ -3,6 +3,7 @@ from __future__ import absolute_import
 from __future__ import division
 from __future__ import print_function
 
+import argparse
 import pdb
 import sys
 from doctest import UnexpectedException
@@ -11,6 +12,31 @@ from _pytest import outcomes
 from _pytest.config import hookimpl
 
 
+def _validate_usepdb_cls(value):
+    try:
+        modname, classname = value.split(":")
+    except ValueError:
+        raise argparse.ArgumentTypeError(
+            "{!r} is not in the format 'modname:classname'".format(value)
+        )
+
+    try:
+        __import__(modname)
+        mod = sys.modules[modname]
+
+        # Handle --pdbcls=pdb:pdb.Pdb (useful e.g. with pdbpp).
+        parts = classname.split(".")
+        pdb_cls = getattr(mod, parts[0])
+        for part in parts[1:]:
+            pdb_cls = getattr(pdb_cls, part)
+
+        return pdb_cls
+    except Exception as exc:
+        raise argparse.ArgumentTypeError(
+            "could not get pdb class for {!r}: {}".format(value, exc)
+        )
+
+
 def pytest_addoption(parser):
     group = parser.getgroup("general")
     group._addoption(
@@ -23,6 +49,7 @@ def pytest_addoption(parser):
         "--pdbcls",
         dest="usepdb_cls",
         metavar="modulename:classname",
+        type=_validate_usepdb_cls,
         help="start a custom interactive Python debugger on errors. "
         "For example: --pdbcls=IPython.terminal.debugger:TerminalPdb",
     )
@@ -35,11 +62,8 @@ def pytest_addoption(parser):
 
 
 def pytest_configure(config):
-    if config.getvalue("usepdb_cls"):
-        modname, classname = config.getvalue("usepdb_cls").split(":")
-        __import__(modname)
-        pdb_cls = getattr(sys.modules[modname], classname)
-    else:
+    pdb_cls = config.getvalue("usepdb_cls")
+    if not pdb_cls:
         pdb_cls = pdb.Pdb
 
     if config.getvalue("trace"):
diff --git a/testing/test_pdb.py b/testing/test_pdb.py
index 43d640614..f9a050b10 100644
--- a/testing/test_pdb.py
+++ b/testing/test_pdb.py
@@ -2,12 +2,14 @@ from __future__ import absolute_import
 from __future__ import division
 from __future__ import print_function
 
+import argparse
 import os
 import platform
 import sys
 
 import _pytest._code
 import pytest
+from _pytest.debugging import _validate_usepdb_cls
 
 try:
     breakpoint
@@ -688,6 +690,23 @@ class TestPDB(object):
         result.stdout.fnmatch_lines(["*NameError*xxx*", "*1 error*"])
         assert custom_pdb_calls == ["init", "reset", "interaction"]
 
+    def test_pdb_custom_cls_invalid(self, testdir):
+        result = testdir.runpytest_inprocess("--pdbcls=invalid")
+        result.stderr.fnmatch_lines(
+            [
+                "*: error: argument --pdbcls: 'invalid' is not in the format 'modname:classname'"
+            ]
+        )
+
+    def test_pdb_validate_usepdb_cls(self, testdir):
+        assert _validate_usepdb_cls("os.path:dirname.__name__") == "dirname"
+
+        with pytest.raises(
+            argparse.ArgumentTypeError,
+            match=r"^could not get pdb class for 'pdb:DoesNotExist': .*'DoesNotExist'",
+        ):
+            _validate_usepdb_cls("pdb:DoesNotExist")
+
     def test_pdb_custom_cls_without_pdb(self, testdir, custom_pdb_calls):
         p1 = testdir.makepyfile("""xxx """)
         result = testdir.runpytest_inprocess("--pdbcls=_pytest:_CustomPdb", p1)