diff --git a/Lib/os.py b/Lib/os.py
index 5f35f4c1a429..12d80eea451a 100644
--- a/Lib/os.py
+++ b/Lib/os.py
@@ -644,7 +644,11 @@ def __init__(self, data, encodekey, decodekey, encodevalue, decodevalue, putenv,
         self._data = data
 
     def __getitem__(self, key):
-        value = self._data[self.encodekey(key)]
+        try:
+            value = self._data[self.encodekey(key)]
+        except KeyError:
+            # raise KeyError with the original key value
+            raise KeyError(key)
         return self.decodevalue(value)
 
     def __setitem__(self, key, value):
@@ -654,9 +658,13 @@ def __setitem__(self, key, value):
         self._data[key] = value
 
     def __delitem__(self, key):
-        key = self.encodekey(key)
-        self.unsetenv(key)
-        del self._data[key]
+        encodedkey = self.encodekey(key)
+        self.unsetenv(encodedkey)
+        try:
+            del self._data[encodedkey]
+        except KeyError:
+            # raise KeyError with the original key value
+            raise KeyError(key)
 
     def __iter__(self):
         for key in self._data:
diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
index 32a67e57c177..e6aae5c6ef08 100644
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -632,6 +632,24 @@ def test_unset_error(self):
             key = 'key='
             self.assertRaises(OSError, os.environ.__delitem__, key)
 
+    def test_key_type(self):
+        missing = 'missingkey'
+        self.assertNotIn(missing, os.environ)
+
+        try:
+            os.environ[missing]
+        except KeyError as err:
+            self.assertIs(err.args[0], missing)
+        else:
+            self.fail("KeyError not raised")
+
+        try:
+            del os.environ[missing]
+        except KeyError as err:
+            self.assertIs(err.args[0], missing)
+        else:
+            self.fail("KeyError not raised")
+
 class WalkTests(unittest.TestCase):
     """Tests for os.walk()."""
 
diff --git a/Misc/NEWS b/Misc/NEWS
index df012b2809d0..af2d056e8caf 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -46,6 +46,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #17702: os.environ now raises KeyError with the original environment
+  variable name (str on UNIX), instead of using the encoded name (bytes on
+  UNIX).
+
 - Issue #16163: Make the importlib based version of pkgutil.iter_importers
   work for submodules. Initial patch by Berker Peksag.