Major depressive disorder is a debilitating condition, with many patients unresponsive to conventional monoaminergic antidepressants. Rapid-acting antidepressants such as ketamine and psilocybin offer promising alternatives, relieving symptoms within hours. Ketamine, an NMDA receptor antagonist, and psilocybin, a serotonergic psychedelic primarily targeting 5-HT_2A receptors, both enhance synaptic plasticity in mood-regulating circuits through distinct mechanisms. This review synthesizes recent clinical and preclinical findings on ketamine and psilocybin, emphasizing their molecular targets, circuit-level effects, and converging downstream pathways. A key shared mechanism involves BDNF–TrkB signaling, which promotes spinogenesis and synaptogenesis critical for sustained antidepressant efficacy. We also discuss 5-HT_2A receptor biased agonism as a potential strategy to dissociate psilocybin’s therapeutic effects from its hallucinogenic actions. By comparing their mechanistic profiles, we identify both overlapping and distinct features that may inform the development of next-generation rapid-acting antidepressants. Understanding how serotonergic, glutamatergic, and neurotrophic systems converge may guide the development of fast-acting, durable, and non-hallucinogenic antidepressants.