<p>This study examined the causal relationships between university students’ social media use (e.g., LINE, Twitter (X), Instagram) and their subjective well-being (SWB) in Japan, considering the effects of gender and academic standing. Between May 2021 and May 2022, a two-wave panel survey of university students enrolled in the Kanto region of Japan was conducted. The cross-lagged effects model analysis clarified the following results: (1) Overall, the amount of time spent using LINE decreased the participants’ SWB, which was significant among first-year students. (2) Female students’ amount of time spent using Instagram tended to improve their SWB. (3) SWB had a negative effect on posting frequency on Twitter overall and among male students. (4) Lower SWB led third-year students to use Twitter longer. Data were collected during the coronavirus disease (COVID-19) pandemic, and further analyses were conducted to clarify whether this affected the results. SWB had marginally negative effects on anxiety toward COVID-19 among male Instagram users and female Twitter users. In contrast, anxiety toward COVID-19 had a marginally weak negative effect on SWB among third-year students who used Twitter. These findings suggest that the relationship with SWB may differ according to the type of social media used. These relationships are not limited to specific periods, such as the COVID-19 pandemic. Additionally, improving the SWB of the younger generations could help prevent excessive Twitter usage. Future education on social media use should consider how social media types affect different causal relationships with SWB among young adults, including the effects of gender and academic standing.</p>