Recent research suggests that the risk of exercise addiction (REA) is primarily predicted by passion. However, this relationship stems from cross-sectional studies. The objective of this 12-week longitudinal investigation was to examine and compare the temporal changes in REA and passion among women and men (N = 149) who just started a new sport activity. To further test their similarities and distinctiveness, we also examined how exercise motivations predict REA and passion. Latent growth modeling showed that REA and passion were high at baseline and showed a slight increase over the 12 weeks. Passion was predicted by several motivational factors, whereas REA was predicted by being female, team sport participation, higher exercise intensity, and identified motivation. These findings suggest that the development of passion and REA over time, both being associated with sport motivations, manifest independent patterns. Consequently, despite the reported strong relationship between the two, they appear to be independent constructs.