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JSONResponse is a response class that serializes data to JSON format. It’s the default response class in FastAPI when you return data from path operations.

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Import

from fastapi.responses import JSONResponse

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Class Signature

class JSONResponse(Response):
    media_type = "application/json"

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Constructor Parameters

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content
Any
default:"None"
The data to be JSON-serialized and returned in the response body. Can be any JSON-serializable type including dicts, lists, Pydantic models, etc.
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status_code
int
default:"200"
The HTTP status code for the response.
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headers
dict | None
default:"None"
Additional HTTP headers to include in the response.
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media_type
str | None
default:"None"
Override the default media type. If not provided, uses application/json.
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background
BackgroundTask | None
default:"None"
Background task to run after returning the response.

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Usage

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Automatic JSON Response

FastAPI automatically returns JSONResponse when you return data: 
from fastapi import FastAPI

app = FastAPI()

@app.get("/items/")
async def read_items():
    return {"item_id": "Foo", "name": "Bar"}

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Explicit JSONResponse

Return JSONResponse directly to control status codes and headers: 
from fastapi import FastAPI
from fastapi.responses import JSONResponse

app = FastAPI()

@app.get("/items/")
async def read_items():
    content = {"item_id": "Foo", "name": "Bar"}
    return JSONResponse(
        content=content,
        status_code=200,
        headers={"X-Custom-Header": "value"}
    )

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Custom Status Code

from fastapi import FastAPI
from fastapi.responses import JSONResponse

app = FastAPI()

@app.post("/items/")
async def create_item():
    return JSONResponse(
        content={"message": "Item created"},
        status_code=201
    )

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Properties

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media_type

The media type for JSON responses: 
JSONResponse.media_type  # "application/json"

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Methods

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render()

Serializes content to JSON bytes: 
def render(self, content: Any) -> bytes:
    # Returns JSON-encoded bytes
This method is called internally by Starlette/FastAPI to encode the response content.

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Notes

  • FastAPI uses Pydantic for JSON serialization when a return type or response model is set, which is faster than custom response classes
  • The default JSON encoder handles common Python types like datetime, UUID, etc.
  • For Pydantic models, use response models instead of manually creating JSONResponse objects

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