A dumbbell assembly includes a base seat having two receiving areas, two weight units respectively received in the receiving areas, and a dumbbell. Each weight unit includes a plurality of weight blocks each having at least one engaging hole. The dumbbell includes a handle, and two weight adjustment modules each including an insertion base, and a plurality of engaging units each including a drive component mounted on the handle, and at least one insertion pin connected to the drive component. The handle is rotatable relative to the weight adjustment modules to drive each engaging unit to move between an engaging state and a disengaging state.