Weather and climate have a significant impact on agriculture. The agricultural sector and crop yield are affected especially by increasing temperature, leading to heat stress on plants. Heat stress is causing overall yield reduction, and unfavorable traits, to plant death. This research is aimed to evaluate the efficiency of applying nano chitosan (Ch. NPs) and Spirulina Platensis extract (Sp. extract) as biostimulants to eliminate heat stress impact in tomato plants during the flowering stage. The application of biostimulants to enhance plant response to heat stress is one of the most effective, economical, and environmental approaches. In a controlled growth room with a vertical farming system, we evaluated the efficacy of foliar application of the (Sp. extract) 15% and (Ch. NPs) 50 ppm in regular growth conditions as well as in mitigating long-term moderate heat stress for 10 days (LTMHS) during the tomato flowering growth stage. Both showed remarkable results in flowering growth. Also, these results are associated with the accumulation of total soluble sugars, Phenolics, and flavonoids in heat-stressed plants. Finally, these findings recommend the application of (Ch. NPs) and (Sp. extract) in stressed conditions to mitigate the negative effects and increase overall plant productivity.