diff --git a/py2http/default_configs.py b/py2http/default_configs.py
index f0c51f9..baab228 100644
--- a/py2http/default_configs.py
+++ b/py2http/default_configs.py
@@ -150,7 +150,14 @@ def bottle_error_handler(error: Exception):
 
 
 def flask_error_handler(error: Exception):
-    raise NotImplementedError()
+    """The ``(body, status, headers)`` a Flask view returns for an error, mapped like
+    the bottle handler (``OPAQUE_ERRORS`` hides the text of a 500)."""
+    message = str(error)
+    status = error_status(error)
+    if status == 500 and os.getenv("OPAQUE_ERRORS", None):
+        message = "Internal server error"
+    status_line = f"400 {type(error).__name__}" if status == 400 else status
+    return {"error": message}, status_line, _status_headers(status)
 
 
 def default_error_handler(error: Exception):
diff --git a/py2http/tests/test_error_statuses.py b/py2http/tests/test_error_statuses.py
index ad43571..e71e9ef 100644
--- a/py2http/tests/test_error_statuses.py
+++ b/py2http/tests/test_error_statuses.py
@@ -63,3 +63,39 @@ def test_aiohttp_error_handler(error, status):
     assert caught.value.status == status
     if status == 500:
         assert "internal detail" not in caught.value.text
+
+
+@pytest.mark.parametrize("error, status", CASES)
+def test_flask_error_handler(error, status, monkeypatch):
+    flask = pytest.importorskip("flask")
+    monkeypatch.delenv("PY2HTTP_WWW_AUTHENTICATE", raising=False)
+    monkeypatch.setenv("PY2HTTP_FRAMEWORK", "flask")
+    from py2http.default_configs import default_error_handler
+
+    app = flask.Flask(__name__)
+
+    @app.route("/fail")
+    def fail():
+        return default_error_handler(error)
+
+    response = app.test_client().get("/fail")
+    assert response.status_code == status
+    assert set(response.get_json()) == {"error"}
+    if status == 400:  # as the bottle handler: the status line names the error
+        assert response.status == f"400 {type(error).__name__}"
+    if status == 401:
+        assert response.headers["WWW-Authenticate"] == "Bearer"
+    else:
+        assert "WWW-Authenticate" not in response.headers
+    if status == 500:
+        assert "internal detail" in response.get_json()["error"]
+
+
+def test_flask_opaque_errors_hides_500_text(monkeypatch):
+    flask = pytest.importorskip("flask")
+    monkeypatch.setenv("OPAQUE_ERRORS", "1")
+    from py2http.default_configs import flask_error_handler
+
+    app = flask.Flask(__name__)
+    app.add_url_rule("/f", "f", lambda: flask_error_handler(KeyError("internal detail")))
+    assert app.test_client().get("/f").get_json() == {"error": "Internal server error"}