Depression, characterized by persistent low mood and anhedonia, represents a prevalent affective disorder imposing substantial medical and economic burdens. Current first-line antidepressants are limited by delayed onset and treatment resistance. Psilocybin (PSI) has re-emerged as a promising therapeutic agent with rapid, sustained antidepressant effects. This systematic review was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 guidelines and prospectively registered with International Prospective Register of Systematic Reviews (PROSPERO) (CRD420261425626). Comprehensive searches of PubMed, Web of Science, and ScienceDirect identified studies published between January 2021 and March 2026 that examined the antidepressant mechanisms and clinical efficacy of PSI. Of 949 records screened, 46 original studies were included. We synthesized evidence across the molecular, cellular, neural circuit, and clinical dimensions. Our findings suggest that psilocin, the active metabolite, may exert antidepressant effects through multilevel mechanisms, including 5-hydroxytryptamine receptor 2A (5-HT2a) receptor activation, brain-derived neurotrophic factor (BDNF)-tropomyosin receptor kinase B(TrkB) signaling, hippocampal neurogenesis, and default mode network functional connectivity remodeling. Clinical trials confirm that single or limited-dose PSI (particularly 25 mg) rapidly and sustainably alleviates symptoms in treatment-resistant depression, with efficacy comparable or superior to that of escitalopram. This multilevel framework bridges mechanistic insights with clinical translation, supporting PSI as a novel therapeutic pathway for depression.
