diff --git a/tests/run-tests.py b/tests/run-tests.py
--- a/tests/run-tests.py
+++ b/tests/run-tests.py
@@ -1731,7 +1731,9 @@ class TextTestRunner(unittest.TextTestRu
                                     'cuser': ('%0.3f' % timesd[tc.name][0]),
                                     'csys': ('%0.3f' % timesd[tc.name][1]),
                                     'start': ('%0.3f' % timesd[tc.name][3]),
-                                    'end': ('%0.3f' % timesd[tc.name][4])}
+                                    'end': ('%0.3f' % timesd[tc.name][4]),
+                                    'diff': result.faildata.get(tc.name, ''),
+                                    }
                             outcome[tc.name] = tres
                     jsonout = json.dumps(outcome, sort_keys=True, indent=4)
                     fp.writelines(("testreport =", jsonout))
diff --git a/tests/test-run-tests.t b/tests/test-run-tests.t
--- a/tests/test-run-tests.t
+++ b/tests/test-run-tests.t
@@ -530,6 +530,7 @@ test for --json
       "test-failure.t": [\{] (re)
           "csys": "\s*[\d\.]{4,5}", ? (re)
           "cuser": "\s*[\d\.]{4,5}", ? (re)
+          "diff": "---.+\+\+\+.+", ? (re)
           "end": "\s*[\d\.]{4,5}", ? (re)
           "result": "failure", ? (re)
           "start": "\s*[\d\.]{4,5}", ? (re)
@@ -538,6 +539,7 @@ test for --json
       "test-skip.t": {
           "csys": "\s*[\d\.]{4,5}", ? (re)
           "cuser": "\s*[\d\.]{4,5}", ? (re)
+          "diff": "", ? (re)
           "end": "\s*[\d\.]{4,5}", ? (re)
           "result": "skip", ? (re)
           "start": "\s*[\d\.]{4,5}", ? (re)
@@ -546,6 +548,7 @@ test for --json
       "test-success.t": [\{] (re)
           "csys": "\s*[\d\.]{4,5}", ? (re)
           "cuser": "\s*[\d\.]{4,5}", ? (re)
+          "diff": "", ? (re)
           "end": "\s*[\d\.]{4,5}", ? (re)
           "result": "success", ? (re)
           "start": "\s*[\d\.]{4,5}", ? (re)
@@ -575,6 +578,7 @@ Test that failed test accepted through i
       "test-failure.t": [\{] (re)
           "csys": "\s*[\d\.]{4,5}", ? (re)
           "cuser": "\s*[\d\.]{4,5}", ? (re)
+          "diff": "", ? (re)
           "end": "\s*[\d\.]{4,5}", ? (re)
           "result": "success", ? (re)
           "start": "\s*[\d\.]{4,5}", ? (re)
@@ -583,6 +587,7 @@ Test that failed test accepted through i
       "test-skip.t": {
           "csys": "\s*[\d\.]{4,5}", ? (re)
           "cuser": "\s*[\d\.]{4,5}", ? (re)
+          "diff": "", ? (re)
           "end": "\s*[\d\.]{4,5}", ? (re)
           "result": "skip", ? (re)
           "start": "\s*[\d\.]{4,5}", ? (re)
@@ -591,6 +596,7 @@ Test that failed test accepted through i
       "test-success.t": [\{] (re)
           "csys": "\s*[\d\.]{4,5}", ? (re)
           "cuser": "\s*[\d\.]{4,5}", ? (re)
+          "diff": "", ? (re)
           "end": "\s*[\d\.]{4,5}", ? (re)
           "result": "success", ? (re)
           "start": "\s*[\d\.]{4,5}", ? (re)