A dumbbell having weight plates onto which magnetically coupled incremental weight plates can be securely mounted comprises a bar, weights at the end regions of the bar, means for retaining the weight plates at the respective ends to define a handle region therebetween, and a sleeve of generally annular cross-section securely mounted about the bar at least one end region axially outward from the weight plate, said sleeve member having (a) a hub portion for accommodating the through-opening of a weight plate, and (b) a flange portion of greater exterior dimension than the hub portion and having at least one magnetically-responsive region for magnetically securing an incremental weight to the dumbbell when said incremental weight plate is mounted about said hub region. In accordance with the invention, an adaptor to convert conventional dumbbells to utilize magnetically-coupled incremental weight plates comprises elements (a) and (b) above.