Inquiry-based out-of-school STEM learning environments, such as offerings from science centers, museums, and out-of-school laboratories, serve as an enrichment to classroom education in many countries. As there is an increasing number of such STEM learning environments in Germany and some other countries and a growing body of research worldwide, it is important to analyze their effects. In this context, student interest is a critical factor in determining the effectiveness of such learning opportunities and the quality of the educational outcomes achieved. Therefore, based on an international comprehensive, systematic literature review of 30 out of 1657 identified studies, various influencing factors that support or hinder interest development are highlighted. Results show that active participation, hands-on activities, and preparation for the visit have strong beneficial effects on interest experience. Out-of-school STEM learning environments are especially suitable for students with little prior knowledge, and they tend to promote interest in areas that are often relatively unpopular among adolescents, depending on their age and gender. The findings also highlight research deficits, particularly with respect to well-defined constructs of interest, and point to useful criteria that informal out-of-school learning environments can apply to promote various forms of interest and thus improve educational programs.