In this paper we examine whether the relationship between transformational leadership and organizational citizenship behaviours (OCBs) is contingent on public service motivation (PSM). We propose that PSM may reduce the motivational influences of transformational leaders’ behaviours on followers’ OCBs in public sector organizations. Using a sample of Mexican employees we tested this proposition with structural equation modelling. Our results show that the motivational effects of transformational leadership were less for public sector followers higher in PSM than for those lower in PSM. A follow‐up study in private sector organizations did not reveal a similar interaction effect. These findings appear consistent with previous research demonstrating that PSM is more aligned to the goals and values of public rather than private sector organizations. Nevertheless, the direct effects of PSM on OCBs remained in the private sector.