Abstract: A bicycle seat security device, for preventing the theft of a removable bicycle seat and seat supporting post from the frame of a bicycle, has a security ring and a mounting device, e.g., a clamp or weld for establishing a connection between the security ring and the bicycle seat post. The ring has an opening for fastening a lock to the ring. A rigid connection between the security ring and the bicycle seat post secures the ring to the post with the opening in the ring oriented in a position that enables a lock to be passed through the opening in the security ring so as to lock the bicycle seat and post to the bicycle frame or other object. The ring is preferably mounted on the post in an upright position so that the center axis of the ring opening is horizontal when the post is upright and on the rear of the post beneath the rear portion of the seat. The ring can have connected to it a bracket or clamp with an opening having a center axis at right angles to the axis of the ring opening.