An adjustment assembly for securing a bicycle seat includes a base adapted to be mounted on top of a seat post and having an arcuate rail formed on two opposed sides of the base, two adjusting seats oppositely and slidably connected to the base. Each adjusting seat has an arcuate track to receive therein a corresponding one of the arcuate rail of the base such that the two adjusting seats are movable relative to the base after the two arcuate rails of the base are received in the corresponding arcuate tracks of the adjusting seats and the bicycle seat sandwiched between the two adjusting seats is able to move along with the two adjusting seats relative to the base to accomplish angle change purpose and a securing element extending through the two adjusting seats to threadingly connect to a nut for securing the bicycle seat sandwiched between the two adjusting seats.