The objective of the present study was to compare a group-mediated cognitive behavioral (GMCB) physical activity intervention with traditional exercise therapy (TRAD) upon select social cognitive outcomes in sedentary knee osteoarthritis (knee OA) patients. A total of 80 patients (mean age = 63.5 years; 84% women) were recruited using clinic and community-based strategies to a 12-month, single-blind, two-arm, randomized controlled trial. Mobility-related self-efficacy, self-regulatory self-efficacy (SRSE), and satisfaction with physical function (SPF) were assessed at baseline, 3, and 12 months. Results of intent-to-treat 2 (Treatment: GMCB and TRAD) x 2 (Time: 3 and 12 month) analyses of covariance yielded significantly greater increases in SRSE and SPF (P < 0.01) relative to TRAD. Partial correlations revealed that changes in SRSE and SPF were significantly related (P < 0.05) to improvements in physical activity and mobility at 3 and 12-months. The GMCB intervention yielded more favorable effects on important social cognitive outcomes than TRAD; these effects were related to improvements in physical activity and mobility.