Merchants in the traditional marketplace are exposed to poor working conditions, such as long working hours, musculoskeletal stress, lack of physical activity, irregular meal times, and nutritional imbalance. This study aimed to develop a community capacity-building program for the wellness of traditional marketplace merchants and examine its effectiveness. A total of 60 merchants from two traditional markets were allocated to an experimental group and control group. The program consisted of four sections: Training wellness leaders in the traditional marketplace, wellness, organizing wellness committee, and promoting wellness partnerships. The program was conducted over 12 weeks. Significant differences were observed in week 12 between the experimental and control groups in leadership (p = 0.010), health knowledge (p < 0.001), health self-efficacy (p = 0.016), wellness (p = 0.001), and community capacity (p = 0.038). The community capacity-building program for the wellness of traditional marketplace merchants was effective in improving marketplace merchants’ leadership, health knowledge, health self-efficacy, wellness, and community capacity. Therefore, it is recommended to actively utilize this community capacity program for market merchants.