diff --git a/dvc/command/diff.py b/dvc/command/diff.py
index 7ec4f24f3..7f03b1c5c 100644
--- a/dvc/command/diff.py
+++ b/dvc/command/diff.py
@@ -1,4 +1,5 @@
 import argparse
+import json
 import logging
 
 from dvc.command.base import CmdBase, append_doc_link
@@ -11,9 +12,16 @@ logger = logging.getLogger(__name__)
 class CmdDiff(CmdBase):
     def run(self):
         try:
-            self.repo.diff(
+            diff = self.repo.diff(
                 self.args.a_ref, self.args.b_ref, target=self.args.target
             )
+            if not any(diff.values()):
+                return 0
+
+            if self.args.json:
+                print(json.dumps(diff))
+                return 0
+
         except DvcException:
             logger.exception("failed to get 'diff {}'")
             return 1
@@ -56,4 +64,10 @@ def add_parser(subparsers, parent_parser):
             "that are under DVC control in the current working space."
         ),
     )
+    diff_parser.add_argument(
+        "--json",
+        help=("Format the output into a JSON"),
+        action="store_true",
+        default=False,
+    )
     diff_parser.set_defaults(func=CmdDiff)
diff --git a/tests/func/test_diff.py b/tests/func/test_diff.py
index ce92caa4b..192b56887 100644
--- a/tests/func/test_diff.py
+++ b/tests/func/test_diff.py
@@ -1,7 +1,10 @@
 import hashlib
+import json
 import os
 import pytest
 
+from dvc.main import main
+
 
 def digest(text):
     return hashlib.md5(bytes(text, "utf-8")).hexdigest()
@@ -126,7 +129,7 @@ def test_directories(tmp_dir, scm, dvc):
     assert dvc.diff(":/init", ":/directory") == {
         "added": [
             {
-                "filename": os.path.dir("dir", ""),
+                "filename": os.path.join("dir", ""),
                 "checksum": "5fb6b29836c388e093ca0715c872fe2a.dir",
             },
             {"filename": os.path.join("dir", "1"), "checksum": digest("1")},
@@ -179,13 +182,19 @@ def test_cli(tmp_dir, scm, dvc):
     pytest.skip("TODO: define output structure")
 
 
-def test_json(tmp_dir, scm, dvc):
-    # result = {
-    #     "added": {...},
-    #     "renamed": {...},
-    #     "modified": {...},
-    #     "deleted": {...},
-    # }
+def test_json(tmp_dir, scm, dvc, capsys):
+    assert 0 == main(["diff", "--json"])
+    assert not capsys.readouterr().out
 
-    # main(["diff", "--json"])
-    pytest.skip("TODO: define output structure")
+    tmp_dir.dvc_gen("file", "text")
+    assert 0 == main(["diff", "--json"])
+    assert (
+        json.dumps(
+            {
+                "added": [{"filename": "file", "checksum": digest("text")}],
+                "deleted": [],
+                "modified": [],
+            }
+        )
+        in capsys.readouterr().out
+    )