The invention relates to a child bicycle seat comprising a seating surface and a backrest and also fixing means for fixing the seat on a bicycle. The child bicycle seat furthermore comprises a belt assembly for securing a child in the seat. The belt assembly comprises two shoulder belt parts which are connected by one end to the backrest and are provided with a closing element. A second closing element which can be connected to the closing element of the shoulder belt parts is connected to the seating surface of the seat and in use lies between the legs of the child. The child bicycle seat is provided with a common tautening device for tautening the belt assembly.