Abstract: An adjustable dumbbell includes a shaft provided with left- and right-handed threads, two tubes sleeved around and engaging threadedly and respectively the left- and right-handed threads, a hollow grip rod sleeved on the tubes, two support units each having upper and lower support seats disposed above and below one of the tubes, and a plurality of weights each having an engaging hole section engageable with a corresponding tube. Two adjustment knob units are respectively connected to two connecting ends of the shaft to turn the shaft and to thereby move the tubes in opposite directions towards or away from the connecting ends. Each tube is movable between the support seats to selectively engage one or more engaging hole sections of the weights.