Sustainable consumption, provision of a clean and healthy environment, as well as improvements to the quality of life of current and future generations, are all integral parts of the sustainable development strategy, which is understood as a compromise between the environmental, economic and social objectives of society. The pro-environmental and pro-social consumer engagement in sustainable consumption may lead to behavioral change, thus contributing to the resolution of current global challenges. Although recently the pro-environmental and pro-social engagement concept has received considerable attention, there is still no consensus on what determines it. Moreover, the recent research is limited to identifying individual factors of this phenomenon. Therefore, the purpose of this study is to explore the factors that determine pro-environmental and pro-social consumer engagement in sustainable consumption. The authors employed a qualitative method approach, conducting semi-structured interviews with consumers engaged in sustainable consumption in Lithuania. The research results reveal that these consumers were affected by cognitive, attitudinal and psychosocial factors. In terms of external factors, consumers named contextual factors for sustainable consumption, social norms and the promotion of sustainable consumption. Research results also show an additional internal factor of perceived responsibility, which means perceived duty as an individual to do good for society and the environment.