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.