Introduction: Internet gaming disorder (IGD) is a growing mental health threat across age groups, but existing literature regarding IGD mainly focuses on student populations. Empirical investigation of the risk and protective factors in adult populations is warranted. This study aimed to fill the research gap by examining whether stress and 3 positive psychology factors (ie, frustration tolerance, mindfulness, and social support) are associated with IGD symptoms in working adults. It was also the first attempt to test the buffering effects of these positive psychology factors on the relationship between stress and IGD vulnerability. Method: This cross-sectional study was conducted in Shenzhen, China. We recruited 327 full-time working Chinese adults (mean age = 31.93 years), who had online gaming experience and voluntarily completed an anonymous questionnaire with DSM-5 criteria to measure their IGD symptoms. Results: Internet gaming disorder symptoms were positively correlated with stress and negatively correlated with the 3 positive psychology factors, among which mindfulness emerged as the most salient protective factor. Moreover, mindfulness, but not frustration tolerance and social support, was found to significantly alleviate the relationship between stress and IGD. Discussion: Our findings provide supportive evidence for the protective and moderating roles of positive psychology variables against IGD among Chinese working adults. Workplace-based prevention programs may take the identified factors into account to help promote individuals' personal resources to mitigate development of IGD.