Purpose: The purpose of this study were to design a new functional splint which supports and controls the position of the trapeziometacarpal (TMC) joint during activities of daily living in patients with TMC osteoarthritis (TMCOA) and to investigate the effectiveness of the splint on pain and function. Methods: Eleven patients with a clinical and radiological diagnosis of TMCOA were included into the study. Information about the progress of TMCOA, principles of joint protection and rationale of splinting was given at the first visit. Patients received the splint at the second visit. The splint held the TMC joint in slight traction and medial rotation, and permitted activities of daily living. Patients used it during activities of daily living for six weeks. Other physiotherapy approaches were not implemented within that period. Assessments were performed at baseline and six weeks later without splint. Pain was assessed with Visual Analogue Scale, grip and pinch strengths were assessed using dynamometer and pinchmeter, functional level was assessed using the Disabilities of the Arm, Shoulder, and Hand (DASH) questionnaire and Michigan Hand Outcome Questionnaire and splint use was assessed using splint diary. Results: Adherence to splint use was high. Pain was significantly decreased after having used the splint for 6 weeks (p<0.05). There was an improvement in functional level (p<0.05). No significant differences were found for grip and pinch strength (p>0.05). Conclusion: We designed a new splint, which can be comfortably used during activities of daily living for patients with TMCOA. The splint was effective in terms of reducing pain and improving function.