A backrest locking mechanism adapted for a backrest and a base of a child safety seat is provided. The backrest locking mechanism includes a locking pin, a locking component and an operating component. The locking pin is disposed on the base. The locking component is disposed on the backrest. A locking hole structure is formed on the locking component. The locking pin is configured to detachably engage with the locking hole structure. The operating component is movably disposed on the backrest or the base. The operating component drives the locking pin and the locking hole structure to engage with or disengage from each other when the operating component is operated. Besides, a child safety seat including the aforementioned backrest locking mechanism is provided.