Fungal diseases are a major threat to the horticultural industry and cause substantial postharvest losses. While secondary metabolites from Trichoderma sp. have been explored for their antifungal properties, limited information exists on the phenolic compounds produced by less studied species like Trichoderma longibrachiatum. In this study, phenolic compounds were extracted from a liquid culture of T. longibrachiatum using various solvents and methods (conventional and ultrasonic-assisted). Phenolic compounds were quantified by spectrophotometry and identified by high-performance liquid chromatography with diode array detection (HPLC-DAD). The antifungal activity against Alternaria alternata and Fusarium oxysporum was determined by mycelial growth inhibition assays, maximum growth rate (µ_max) by the Gompertz equation, and spore germination tests. Although no significant differences (p ≥ 0.05) were found between the extraction methods, the type of solvent significantly influenced the phenolic content (p ≤ 0.05). Extraction with 70% ethanol showed the highest content of phenolic compounds and flavonoids. More than eight phenolic compounds were detected. Further, this is the first report of the phenolics ferulic, chlorogenic and p-coumaric acids identification in T. longibrachiatum, along with flavonoids such as epicatechin and quercetin, among others. The 70% ethanolic extracts notably inhibited the mycelial growth of A. alternata and F. oxysporum, reducing their maximum growth rate by 1.5 and 1.4 mm/h, respectively. Furthermore, p-coumaric and ferulic acids significantly inhibited spore germination of both pathogens, with a minimum inhibitory concentration (MIC) of 1.5 mg/mL and a minimum fungicidal concentration (MFC) of 2 mg/mL. These findings demonstrate the potential of T. longibrachiatum and its phenolic compounds as viable alternatives for biological control in horticulture and postharvest disease management.