WeChat is a highly popular social media in China and many other Asian countries, but little is known about its effectiveness in facilitating international students' academic and social functioning. Hence, the present study aimed to examine causal or reciprocal relationships among WeChat usage intensity, behavioral engagement in academic learning (BE-academic) and in local social activities (BE-social), and academic and social adjustment. To this end, we employed a three-wave longitudinal design among international students in China with data collected at three times (i.e., Time 1 data collected in December of 2020, Time 2 data collected in March of 2021, and Time 3 data collected in June of 2021). Results based on the cross-lagged panel analyses indicated that in academic domains, WeChat usage intensity positively predicted longitudinal changes in BE-academic and academic adjustment positively predicted longitudinal changes in WeChat usage intensity across Time 1 and Time 3. In social domains, WeChat usage intensity positively predicted longitudinal changes in BE-social and social adjustment, and BE-social positively predicted longitudinal changes in social adjustment from Time 1 to Time 2. Additionally, the reverse effects of social adjustment on WeChat usage intensity were revealed across Time 1 and Time 3.