In recent times, clothing upcycling has emerged as a sustainable solution to tackling textile waste. Despite its popularity, there remains a limited understanding of the factors driving individuals’ upcycling behavior. This study delves into the determinants of clothing upcycling behavior within the Japanese population. Administered via an online survey involving 433 participants in Japan, this research utilized structural equation modeling to assess a theoretical model that integrates personal norms, attitudes, social norms, perceived behavioral control, and intention within the theory of planned behavior. The outcomes reveal that both personal norms and perceived behavioral control exert a substantial influence on individuals’ intentions to participate in upcycling activities. Interestingly, the impact of attitude on upcycling intentions is moderate, while subjective norms surprisingly showed no significant influence. These findings contribute to the pro-environmental behavior literature and can guide the development of focused interventions that promote sustainable fashion consumption through upcycling.