With rising food insecurity being a global challenge, the strain on food systems is further exacerbated by post-harvest losses occasioned by fungal attacks which result in food contamination. In this case, there is an urgent need to develop new and sustainable bio-based antifungal agents that can control pathogenic fungi during pre-harvest and post-harvest. For this reason, chitosan (Cs), which has been shown to have antifungal properties, is a promising antifungal agent owing to its non-toxicity and biodegradability. This study therefore sought to evaluate the potential applications of dialdehyde chitosan (OCs) synthesized from Cs isolated from Hermetia illucens in the inhibition of two fungal strains that have been found in food matrices. Cs was chemically isolated from Hermetia illucens (black soldier fly) followed by periodate oxidation to introduce carbonyl groups (C = O) in its structure as confirmed by Fourier transform infrared (FTIR) spectroscopy. X-ray fluorescence (XRF), energy-dispersive X-ray (EDX) spectroscopy, and thermogravimetric analysis (TGA) data indicated the removal of extractives, purity, and thermal properties of both Cs and OCs while scanning electronic microscopy (SEM) images of Cs were rougher than OCs and indicative of successful oxidation. Additionally, the Viscometer indicated successful oxidation of Cs evident by a decrease in fluidity of OCs. From the results obtained, the degree of oxidation of the aldehydes increases while the viscosity also decreases with an increase in a molar ratio of KIO₄/Cs and the temperature. Using the plate count method, the OCs had significant inhibitory activity against Aspergillus brasiliensis and Candida albicans when compared to Cs.