diff --git a/_pytest/assertion/rewrite.py b/_pytest/assertion/rewrite.py
index 92deb6539..db3674930 100644
--- a/_pytest/assertion/rewrite.py
+++ b/_pytest/assertion/rewrite.py
@@ -179,11 +179,13 @@ class AssertionRewritingHook(object):
         The named module or package as well as any nested modules will
         be rewritten on import.
         """
-        already_imported = set(names).intersection(set(sys.modules))
-        if already_imported:
-            for name in already_imported:
-                if name not in self._rewritten_names:
-                    self._warn_already_imported(name)
+        already_imported = (set(names)
+                            .intersection(sys.modules)
+                            .difference(self._rewritten_names))
+        for name in already_imported:
+            if not AssertionRewriter.is_rewrite_disabled(
+                    sys.modules[name].__doc__ or ""):
+                self._warn_already_imported(name)
         self._must_rewrite.update(names)
 
     def _warn_already_imported(self, name):
@@ -635,7 +637,8 @@ class AssertionRewriter(ast.NodeVisitor):
                       not isinstance(field, ast.expr)):
                     nodes.append(field)
 
-    def is_rewrite_disabled(self, docstring):
+    @staticmethod
+    def is_rewrite_disabled(docstring):
         return "PYTEST_DONT_REWRITE" in docstring
 
     def variable(self):
diff --git a/_pytest/pytester.py b/_pytest/pytester.py
index 0b25d839b..ee7ca24cd 100644
--- a/_pytest/pytester.py
+++ b/_pytest/pytester.py
@@ -347,7 +347,7 @@ class RunResult:
     :stdout: :py:class:`LineMatcher` of stdout, use ``stdout.str()`` to
        reconstruct stdout or the commonly used ``stdout.fnmatch_lines()``
        method
-    :stderrr: :py:class:`LineMatcher` of stderr
+    :stderr: :py:class:`LineMatcher` of stderr
     :duration: duration in seconds
 
     """
diff --git a/changelog/2995.bugfix b/changelog/2995.bugfix
new file mode 100644
index 000000000..7a3dde4c8
--- /dev/null
+++ b/changelog/2995.bugfix
@@ -0,0 +1 @@
+``PYTEST_DONT_REWRITE`` is now checked for plugins too rather than only for test modules.
diff --git a/testing/test_assertrewrite.py b/testing/test_assertrewrite.py
index 0e22c6dac..77cfd2900 100644
--- a/testing/test_assertrewrite.py
+++ b/testing/test_assertrewrite.py
@@ -128,6 +128,16 @@ class TestAssertionRewrite(object):
             assert len(m.body) == 1
         assert m.body[0].msg is None
 
+    def test_dont_rewrite_plugin(self, testdir):
+        contents = {
+            "conftest.py": "pytest_plugins = 'plugin'; import plugin",
+            "plugin.py": "'PYTEST_DONT_REWRITE'",
+            "test_foo.py": "def test_foo(): pass",
+        }
+        testdir.makepyfile(**contents)
+        result = testdir.runpytest_subprocess()
+        assert "warnings" not in "".join(result.outlines)
+
     def test_name(self):
         def f():
             assert False