An adjustable dumbbell assembly includes a dumbbell mechanism having two or more weight members engaged in a receptacle, and a handle bar engaged between the weight members, two catches slidably engaged in the handle bar and extendible out and engageable with the weight members for detachably attaching the weight members to the handle bar, two followers slidably engaged in the receptacle and each having a protrusion for engaging with the catches and for moving the catches to engage with the weight members, and for detachably attaching the weight members to the handle bar, and an actuating member engaged with the followers for actuating the followers to move the catches to engage with the weight members.