A child safety seat includes a seat shell including a backrest that has a lengthwise axis, a headrest assembled with the seat shell and movable along the lengthwise axis of the backrest, and a headrest adjustment mechanism. The headrest adjustment mechanism includes two tube segments affixed with the backrest, a mount frame affixed with the headrest, two latches and a driving part. The mount frame is movable with the headrest along the tube segments. The two latches are assembled with the mount frame, wherein the latches are operable to move transversally relative to the headrest to respectively engage with the two tube segments to lock the headrest in position, and to respectively disengage from the tube segments to allow adjustment of the headrest. The driving part is connected with the two latches, and is movable along the lengthwise axis to cause the latches to respectively disengage from the tube segments.