Previous research investigated the impact of psychosocial predictors (e.g. attitude, social and moral norm, perceived behavioural control, intention) on sustainable clothing purchasing behaviour. To date, no studies considered whether pro-environmental self-identity moderates the effects of these predictors on behaviour. In this study, we adopted an intrapersonal approach to examine the effects of pro-environmental self-identity on different behaviours and a longitudinal design to assess the moderating role of pro-environmental self-identity in predicting intentions and behaviours, controlling for past behaviour. 250 participants completed a first questionnaire designed to measure the Theory of Planned Behaviour constructs related to three different sustainable clothing purchase behaviours. One month later, the participants completed a second questionnaire in which they assessed their self-assessment of these behaviours. The results showed that for all behaviours, affective attitude influenced behaviour positively, while cognitive attitude had a negative influence. The influence of previous behaviours increased with a decrease in pro-environmental self-identity.