Background: While psychosocial risk factors of Internet gaming disorder (IGD) were well-documented, there is very limited research, particularly with longitudinal design, on protective factors against IGD. Based on previous findings, this study identified purpose in life and social support as potential protective factors and tested if they actually lowered ones’ IGD symptoms over time among youths. Method: Two hundred and eighty-three university students, recruited at a public university in Macao, China, participated in an anonymous questionnaire survey in 2016 (W1) and a oneyear follow-up study (W2). Results: As hypothesized, social support and purpose in life had significantly negative correlations with IGD in both waves (ps < .05). Results of cross-lagged panel analysis showed that higher purpose in life at W1 (p < .05), but not social support, significantly predicted the lower level of IGD at W2. The finalized cross-lagged model also supported a bidirectional association between purpose in life and social support (ps < .05). Conclusions: Our findings showed that purpose in life was a salient protective factor against IGD, while social support and purpose in life had a reciprocal relationship. Strategies to cultivate and search for purpose / meaning in life should be considered in IGD intervention for Chinese youths.