diff --git a/Lib/inspect.py b/Lib/inspect.py
index 6ca91e401a5eae..18bed9028ed8ab 100644
--- a/Lib/inspect.py
+++ b/Lib/inspect.py
@@ -864,7 +864,12 @@ def findsource(object):
         lnum = object.co_firstlineno - 1
         pat = re.compile(r'^(\s*def\s)|(\s*async\s+def\s)|(.*(?<!\w)lambda(:|\s))|^(\s*@)')
         while lnum > 0:
-            if pat.match(lines[lnum]): break
+            try:
+                line = lines[lnum]
+            except IndexError:
+                raise OSError('lineno is out of bounds')
+            if pat.match(line):
+                break
             lnum = lnum - 1
         return lines, lnum
     raise OSError('could not find code object')
diff --git a/Lib/test/test_inspect.py b/Lib/test/test_inspect.py
index 05485a0bc05754..d9e26a30336676 100644
--- a/Lib/test/test_inspect.py
+++ b/Lib/test/test_inspect.py
@@ -710,6 +710,17 @@ def test_findsource_without_filename(self):
             self.assertRaises(IOError, inspect.findsource, co)
             self.assertRaises(IOError, inspect.getsource, co)
 
+    def test_findsource_with_out_of_bounds_lineno(self):
+        mod_len = len(inspect.getsource(mod))
+        src = '\n' * 2* mod_len + "def f(): pass"
+        co = compile(src, mod.__file__, "exec")
+        g, l = {}, {}
+        eval(co, g, l)
+        func = l['f']
+        self.assertEqual(func.__code__.co_firstlineno, 1+2*mod_len)
+        with self.assertRaisesRegex(IOError, "lineno is out of bounds"):
+            inspect.findsource(func)
+
     def test_getsource_on_method(self):
         self.assertSourceEqual(mod2.ClassWithMethod.method, 118, 119)
 
diff --git a/Misc/NEWS.d/next/Library/2020-12-03-22-22-24.bpo-17735.Qsaaue.rst b/Misc/NEWS.d/next/Library/2020-12-03-22-22-24.bpo-17735.Qsaaue.rst
new file mode 100644
index 00000000000000..655781e3d2eddb
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-12-03-22-22-24.bpo-17735.Qsaaue.rst
@@ -0,0 +1,4 @@
+:func:`inspect.findsource` now raises :exc:`OSError` instead of
+:exc:`IndexError` when :attr:`co_lineno` of a code object is greater than the
+file length. This can happen, for example, when a file is edited after it was
+imported.  PR by Irit Katriel.