This study evaluates the effectiveness of Trichoderma spp. in decomposing watermelon rind to produce bio-organic fertilizer. Response Surface Methodology (RSM) with Central Composite Design (CCD) was employed to optimize the cultivation conditions for Brassica juncea using the composted fertilizer. Results demonstrated that Trichoderma spp. significantly accelerated decomposition and improved fertilizer quality. Among the tested parameters, composting duration and fertilization rate significantly influenced plant growth, particularly leaf length and width, while microbial inoculant concentration had no notable effect. Microbial analysis confirmed that Coliform, E. coli, and Salmonella levels were below regulatory limits (ND 108/2017/ND-CP), with none detected in fertilizers produced with Trichoderma spp.. RSM optimization identified the optimal composting conditions as 18 days, 4 wt.% Trichoderma spp., and 50 v/v% fertilizer volume fraction. A high-accuracy mathematical model was developed to predict plant growth responses, effectively forecasting the impact of composting parameters on crop development. These findings highlight the potential of Trichoderma-based bio-organic fertilizers in sustainable agriculture.