ABSTRACT Physical activity (PA) is critical to health, but PA level declines dramatically in young adulthood. This study aims to compare the effectiveness of different social gamification designs (i.e. cooperation and competition) in mobile fitness applications among occasional exercisers among young adults. A mobile fitness application featuring competitive, cooperative, or individualistic gamification was designed and their effectiveness in promoting PA behaviours and socio-cognitive factors among occasional exercisers was compared through a four-week (i.e. one week for baseline and three weeks for use) field experiment involving 49 participants. The participants used one of the gamified applications with one or more friends in daily life during the experimental period. The results showed that cooperation and competition stimulate different types of social support: cooperation improves companion support, whereas competition improves appraisal support. Both gamification significantly increased daily steps in the first week, but after three-week usage, the increase of the cooperation groups reclined, whereas that of the competition groups was maintained. The application usage in cooperation also declined significantly after three weeks, while that of the other two conditions did not change significantly. The current study provided insights into gamification design for occasional exercisers among Chinese young adults.