Career adaptability and career insecurity in the association between university career support and mental wellbeing among Chinese undergraduates

University students approaching graduation must manage academic demands while forming career plans under uncertain employment conditions, yet the psychological relevance of university career support remains incompletely understood. This cross-sectional study examined whether perceived university career support was associated with mental well-being through career adaptability and whether this indirect association varied with career insecurity. The analytic sample included 800 third- and fourth-year undergraduates from five public universities in Zhejiang Province, China. Covariate-adjusted HC3 regressions showed that career support was positively associated with career adaptability (b = 0.366, p < 0.001), career adaptability was positively associated with well-being (b = 8.431, p < 0.001), and career insecurity was negatively associated with well-being (b = −4.307, p < 0.001). The adaptability-by-insecurity interaction was significant under individual-level HC3 inference (b = 1.839, p = 0.038), and the moderated-mediation index was 0.673 (95% bootstrap CI 0.043–1.319). However, a Webb-weight wild cluster bootstrap at the university level did not confirm the interaction ( p = 0.195), and a leave-one-university-out jackknife interval for the moderated-mediation index included zero. The main associations were evident in the covariate-adjusted models, whereas the moderation findings should be considered exploratory.