University campuses in Southwest Nigeria offer integrated green spaces amid rapid urbanization and academic stressors, yet the mechanisms linking perceived campus environments to student mental health remain underexplored locally. This cross-sectional study examined the mediating role of nature connectedness in the relationship between perceived campus environmental qualities and psychological well-being among 767 undergraduate students from five institutions (University of Ibadan, Obafemi Awolowo University, Federal University of Technology Akure, Adeleke University, and Ekiti State University). Participants completed adapted measures of perceived greenness (5 items), perceived restorativeness (6-item subset from Hartig et al.'s PRS), nature connectedness (NR-6), and well-being (WHO-5 Well-Being Index). Structural equation modeling (path analysis) tested the hypothesized model, yielding excellent fit (CFI = 0.98, RMSEA = 0.038). Results showed significant positive direct effects of perceived greenness (β = 0.244) and restorativeness (β = 0.373) on well-being, and both on nature connectedness (β = 0.398 and 0.460, respectively). Nature connectedness strongly predicted well-being (β = 0.419) and partially mediated the greenness–well-being (indirect effect = 0.143, 61% of total) and restorativeness– well-being (indirect effect = 0.135, 39% of total) relationships. These findings extend Attention Restoration Theory and Stress Reduction Theory to tropical Nigerian campuses, highlighting nature connectedness as a key psychological pathway. Implications support biophilic campus design to enhance restorative green infrastructure, fostering resilience and mental health amid rising psychological challenges.