Little is known about the effectiveness of self-determination theory (SDT), a representative motivational theory, on exercise domain in older adults. This feasibility study used quantitative and qualitative approaches to evaluate the effectiveness of a 13-month group exercise program applying SDT-based motivational strategies on exercise adherence, physical fitness, and quality of life, and to explore factors affecting exercise adherence in South Korean older adults (N = 18). Exercise attendance rate was high (82.52%). There were significant differences in aerobic endurance (p < .001), lower body strength (p < .05), dynamic balance (p < .001), and perceived social functioning (p < .05) at 13 months compared with baseline. Factors affecting exercise adherence were related to the SDT-based motivational strategies. These results support the importance of health professionals applying SDT-based motivational strategies to exercise programs to help facilitate motivation for participation and to promote physical fitness and quality of life in older adults.