An adjustable dumbbell system includes a handle including a grip, two hanging rods at two ends of the grip and multiple through holes at each hanging rod, multiple weights, each weight defining an elongated top slot and a bottom opening and having a scissor hook mounted therein, the scissor hook including a hook unit, an actuation unit drivable by an external force to open or close the hook unit and elastic members adapted to hold the hook unit in the close position, a holder base for accommodating the weights, and a selector including multiple control units that are attached to the bottom openings of the weights when the weights are accommodated in the holder base and a selector rotatable to move the control units in driving the actuation units of the scissor hooks of the weight to open or close the hook units of the respective scissor hooks.