diff --git a/mypy/error_formatter.py b/mypy/error_formatter.py
index a9a95978f94a..ffc6b6747596 100644
--- a/mypy/error_formatter.py
+++ b/mypy/error_formatter.py
@@ -29,7 +29,7 @@ def report_error(self, error: "MypyError") -> str:
                 "message": error.message,
                 "hint": None if len(error.hints) == 0 else "\n".join(error.hints),
                 "code": None if error.errorcode is None else error.errorcode.code,
-                "is_note": error.is_note,
+                "severity": error.severity,
             }
         )
 
diff --git a/mypy/errors.py b/mypy/errors.py
index 9f1665fedc31..7a937da39c20 100644
--- a/mypy/errors.py
+++ b/mypy/errors.py
@@ -1296,14 +1296,14 @@ def __init__(
         column: int,
         message: str,
         errorcode: ErrorCode | None,
-        is_note: bool = False,
+        severity: Literal["error", "note"],
     ) -> None:
         self.file_path = file_path
         self.line = line
         self.column = column
         self.message = message
         self.errorcode = errorcode
-        self.is_note = is_note
+        self.severity = severity
         self.hints: list[str] = []
 
 
@@ -1326,14 +1326,14 @@ def create_errors(error_tuples: list[ErrorTuple]) -> list[MypyError]:
             error = latest_error_at_location.get(error_location)
             if error is None:
                 # This is purely a note, with no error correlated to it
-                error = MypyError(file_path, line, column, message, errorcode, is_note=True)
+                error = MypyError(file_path, line, column, message, errorcode, severity="note")
                 errors.append(error)
                 continue
 
             error.hints.append(message)
 
         else:
-            error = MypyError(file_path, line, column, message, errorcode)
+            error = MypyError(file_path, line, column, message, errorcode, severity="error")
             errors.append(error)
             error_location = (file_path, line, column)
             latest_error_at_location[error_location] = error
diff --git a/test-data/unit/outputjson.test b/test-data/unit/outputjson.test
index 72d36a50415d..43649b7b781d 100644
--- a/test-data/unit/outputjson.test
+++ b/test-data/unit/outputjson.test
@@ -18,7 +18,7 @@ def foo() -> None:
 
 foo(1)
 [out]
-{"file": "main", "line": 5, "column": 0, "message": "Too many arguments for \"foo\"", "hint": null, "code": "call-arg", "is_note": false}
+{"file": "main", "line": 5, "column": 0, "message": "Too many arguments for \"foo\"", "hint": null, "code": "call-arg", "severity": "error"}
 
 [case testOutputJsonWithHint]
 # flags: --output=json
@@ -39,6 +39,6 @@ foo('42')
 def bar() -> None: ...
 bar('42')
 [out]
-{"file": "main", "line": 12, "column": 12, "message": "Revealed type is \"Overload(def (), def (x: builtins.int))\"", "hint": null, "code": "misc", "is_note": true}
-{"file": "main", "line": 14, "column": 0, "message": "No overload variant of \"foo\" matches argument type \"str\"", "hint": "Possible overload variants:\n    def foo() -> None\n    def foo(x: int) -> None", "code": "call-overload", "is_note": false}
-{"file": "main", "line": 17, "column": 0, "message": "Too many arguments for \"bar\"", "hint": null, "code": "call-arg", "is_note": false}
+{"file": "main", "line": 12, "column": 12, "message": "Revealed type is \"Overload(def (), def (x: builtins.int))\"", "hint": null, "code": "misc", "severity": "note"}
+{"file": "main", "line": 14, "column": 0, "message": "No overload variant of \"foo\" matches argument type \"str\"", "hint": "Possible overload variants:\n    def foo() -> None\n    def foo(x: int) -> None", "code": "call-overload", "severity": "error"}
+{"file": "main", "line": 17, "column": 0, "message": "Too many arguments for \"bar\"", "hint": null, "code": "call-arg", "severity": "error"}