The coronavirus disease (COVID-19) outbreak is a threat to adolescents’ mental health and livelihoods, and lowers their subjective well-being (SWB). Expanding on previous literatures, this study examined whether internet use time was related to SWB and whether this relationship was mediated by problematic internet use (PIU) and self-esteem during the COVID-19 outbreak. In Taiwan, the COVID-19 epidemic broke out in February, 2020. During March 2 to 27, this study recruited a total of 1,060 adolescents from junior high schools by both stratified and cluster sampling, and administered a comprehensive investigation. The results displayed that SWB was significantly and negatively predicted by internet use time. PIU fully mediated the relationship. Moreover, PIU predicted a decrease of self-esteem, which played a full mediation role between PIU and SWB. The results provide evidence in explaining how increased internet use time is associated with a greater level of PIU, which relates to lower self-esteem, correlating with lower SWB in adolescents. This study can provide reference to mental health organizations and educational agencies to design appropriate SWB promotion programs for the junior high school population in terms of epidemic prevention.