Postharvest soft rot is one of the most destructive diseases affecting Barbados cherry (Malpighia glabra L.), causing substantial losses in fruit quality and marketability in Dong Thap Province, Viet Nam. This study identified Gilbertella persicaria as the causal agent of postharvest soft rot through morphological characterization and ITS rDNA sequence analysis, and evaluated the inhibitory efficacy of chitosan and selected plant extracts under both in vitro and in vivo conditions. In vitro assays demonstrated that chitosan at concentrations of 0.6–1.0% completely inhibited mycelial growth and markedly suppressed spore production, whereas garlic extract (6–8%) exhibited strong but short-lived antifungal activity. Under in vivo conditions, chitosan at 0.8–1.0% significantly reduced disease incidence (32.5–37.5%) and lesion area (8.9–16.6%) compared with untreated controls. These findings demonstrate the potential of chitosan as an eco-friendly and effective alternative for managing postharvest soft rot in Barbados cherry.