We present a research model of extra-role behavior, integrating the literature on public service motivation (PSM) with stewardship theory. We propose that the extent to which volunteers perform activities that go beyond role prescription depends on the individuals PSM. Our research model is further grounded in the notion that a stewardship-oriented organizational culture serves as a means to enhance the effect of PSM on extra-role behavior. We empirically test this research model in a sample of 475 Austrian and German volunteer firefighters and provide evidence that PSM positively relates to extra-role behavior at the 10% significance level. Furthermore, firefighters commitment to the occupation and organization positively moderates the relationship between PSM and extra-role behavior. We also find that stewardship-like characteristics, such as a motivating work design and perceived organizational support, directly relate to extra-role behavior. Implications for research and practice are discussed, focusing on the relevance of these insights for volunteerism research and management.