This study aimed to investigate the role of self-identity, defined as salient and enduring aspects of one's self-perception (Sparks, 2000), in relation to adolescent physical activity (PA) intentions within the Theory of Planned Behaviour (TPB). School students aged 12 to 18 from two cultural groups (Estonia and Spain) completed measures of attitudes, subjective norms, perceived behavioural control, self-identity, and intentions for PA. Four weeks later participants completed self-reported measures of PA. A structural equation model showed that self-identity had a direct effect on PA intention (beta = 0.33, p less than 0.01) and PA behaviour (beta = 0.31, p less than 0.01). The model accounted for 45% of the variance of PA behaviour from which 4% can be attributed to self-identity. The effect of self-identity on PA was also partially mediated via intention. The multi-group comparison between two cultural groups indicated that no invariances existed between the models of the observed samples. (Contains 2 tables and 1 figure.)