Depression is a significant global health challenge, imposing severe social and economic burdens. Rosa multiflora fructus possesses a range of biological properties; however, its role in reducing stress hormone-induced depression remains unclear. This study examined the antidepressant effects of Rosa multiflora fructus extract (RMFE) on corticosterone (CORT)-induced depressive-like behaviors in mice and explored the underlying molecular mechanisms. ICR mice were administered CORT (40 mg/kg/day, i.p.) for 8 weeks to induce depressive-like behaviors. They were then treated with RMFE (100 or 300 mg/kg/day, p.o.) or selegiline (10 mg/kg/day, p.o.) as a positive control. RMFE treatment significantly alleviated depressive-like behaviors, as evaluated by the tail suspension and forced swimming tests. Furthermore, RMFE reduced hippocampal astrocyte hypertrophy and lowered the protein and mRNA expression of monoamine oxidase A and B (MAOA and MAOB). Phytochemical analysis using UPLC-MS/MS identified five flavonoids in RMFE, including ellagic acid (EA), hyperoside, isoquercetin, quercetin, and quercetin-3-glucuronide. In silico molecular docking revealed that these phenolic compounds interact with MAOA and MAOB, with EA exhibiting the strongest binding affinity. Consistently, EA administration effectively alleviated depressive-like behaviors and increased monoamine neurotransmitter levels in CORT-treated mice. These results suggest that RMFE exerts antidepressant effects by inhibiting MAO, restoring monoamine levels, and modulating hippocampal astrocytic hypertrophy, with EA identified as the primary bioactive compound driving its efficacy.