It has been proposed that ligamentous laxity promotes osteoarthritis of the thumb carpometacarpal joint. However, quantitative measurements of laxity for this joint have not been correlated with its incidence of osteoarthritis nor with other measures of degenerative changes such as joint congruence, surface remodeling, cartilage mechanical properties or biochemical composition. The goal of this study was to design and build a device for testing the laxity of the carpometacarpal joint, with the long-term aim of advancing our understanding of the etiology of osteoarthritis in this joint. This laxity tester has the ability to accurately measure the force-displacement response along the joint’s dorsal-volar, radial-ulnar, and distraction-reduction directions, as well as the torque-rotation response in pronation-supination. The compliance of the tester was evaluated and laxity curves were generated for a sample thumb carpometacarpal joint.