<p>In this study, we investigated the development of gum Arabic (GA)-based composite coatings functionalized with keratin (K) and cinnamon oil (CO) to enhance the physicochemical, nutritional, and microbial stability of tomatoes. Coating formulations containing GA-K-CO_0.1 (27:3:0.1, v/v/v) and GA-K-CO_0.5 (27:3:0.5, v/v/v) were developed and applied as coatings. Mature green tomatoes were treated with GA-K-CO_0.1 and GA-K-CO_0.5 and stored at 27.5&#xa0;°C and 80.7% relative humidity (RH) for 20 days, with quality assessments at 5-day intervals. Formulations with a GA-K-CO_0.1 concentration extended the shelf life of tomatoes by up to 5 days under ambient conditions. This was achieved by significantly reducing weight loss by 31.9%, decay incidence by 33.3%, and exhibiting significant antifungal activity (<i>p</i> &lt; 0.05) compared with the uncoated control, which was coated with distilled water. The tomatoes coated with GA-K-CO_0.5 showed about a 6.21% retention in moisture content, total soluble solids, and a rise of roughly 60% in antioxidant capacity compared with control group. The formulation with a lower level of cinnamon oil in GA-K-CO_0.1 showed a significant (<i>p</i> &lt; 0.05) retention in β-carotene and lycopene levels, and further enhanced nutritional and visual quality, as well as its potential to inhibit bacterial and fungal growth. The gum Arabic-based coating functionalized with keratin and cinnamon oil presents a novel and sustainable method for postharvest tomato preservation, holding considerable promise to lower losses and improve food quality in supply chains.</p>