Lacking regular physical activity is a prevailing issue of our society causing high costs in health care, productivity losses, and million deaths. To counteract this problem, fitness apps are increasingly considered as a means to motivate individuals towards more physical activity. However, many people discontinue their fitness app use. User stories point to a ‘darker side’ of fitness apps indicating adverse effects on users’ competence feelings leading to fitness app discontinuance. To better understand the positive and detrimental motivational effects, this paper draws on self-determination theory and motivational affordances explicating how self-monitoring, rewards, and social recognition affordances of fitness apps affect competence needs and fitness app continuance decisions in consequence. Empirical validation with 283 fitness app user reveals the positive and negative motivational effects. Thereby, this study contributes to research with a motivation-based explanation of fitness app continuance, explicates the ‘darker side’ of fitness apps, and explains the need-relevant characteristics of motivational affordances. The results guide the application and tailoring of motivational affordances in fitness apps.