INTRODUCTION: China is amidst a sexual revolution, with changing sexual practices and behaviors. Sex-seeking mobile phone applications (gay apps) that allow multiple people to meet up quickly may facilitate group sex. This study was therefore undertaken to evaluate group sex among Chinese MSM and to better understand factors associated with group sex. METHODS: An online survey was conducted from September-October 2014, collecting data on socio-demographics, sexual behaviors, use of gay apps and occurrence of group sex among Chinese MSM. Univariate and multivariable logistic regressions were used to compare group sex and non-group sex participants. RESULTS: Of the 1,424 MSM, the majority were under 30 years old (77.5%), unmarried (83.9%), and were gay apps users (57.9%). Overall, 141 (9.9%) participants engaged in group sex in the last 12 months. Multivariate analyses showed that men living with HIV, engaged in condomless anal intercourse with men, and used gay apps were more likely to engage in group sex, with adjusted ORs of 3.74 (95% CI 1.92-7.28), 2.88 (95% CI 2.00-4.16) and 1.46 (95% CI: 1.00-2.13), respectively. Among gay app users, the likelihood of group sex increases with the number of sex partners and the number of sex acts with partners met through a gay app. CONCLUSIONS: Chinese MSM who engage in group sex are also more likely to engage in other risky sexual behaviors, and gay app use may facilitate group sex. Further research is needed among MSM who engage in group sex in order to target interventions and surveillance.