The present application relates to the field of sports equipment, in particular to a weight-adjustable dumbbell and a dumbbell assembly. The weight-adjustable dumbbell comprises a dumbbell grip bar assembly and dumbbell plate groups connected to two ends of the dumbbell grip bar assembly; each dumbbell plate group comprises at least two dumbbell plates; the dumbbell grip bar assembly comprises a rotary plate selecting mechanism used for loosening or selecting at least one dumbbell plate. The present application has the effect of conveniently adjusting the weight of the dumbbell.