<p>Social media is frequently regarded as a means of coping, helping individuals detach from reality and fulfill unmet needs for social connection. However, research indicates that problematic engagement with social media can persist even when users experience relatedness need frustration (RNF) in online spaces. Grounded in the cognitive motivational relational theory (CMRT) and self-determination theory (SDT), this study explores how RNF during social media use contributes to problematic social media use (PSMU) through Fear of Missing Out (FoMO) and the moderating role of mindfulness. The study involved 304 Chinese university students (mean age = 20.59, 59.54% female), divided into a frustrated group and a neutral group, using the scenario-based visualization method to manipulate an online social ostracism event to establish experimental conditions for the two groups. The results showed that online RNF affected PSMU through FoMO in the frustrated group, with mindfulness moderating the relationship between online RNF and FoMO. In the neutral group, the relationships between FoMO and online RNF, online RNF and PSMU, as well as the moderating effect of mindfulness, were all non-significant. These findings indicate that when people feel frustrated, cyberostracism serves as a stressor. In response, individuals may use PSMU as a way to deal with the unease caused by FoMO. Without the stressor, there is no need for coping, and RNF becomes a regular part of daily online life.</p>