Heat stress in poultry is a serious concern, affecting their health and productivity. To effectively address the issue of heat stress, it is essential to include antioxidant-rich compounds in the poultry diet to ensure the proper functioning of the redox system. Microalgae (Spirulina platensis) are rich in antioxidants and have several health benefits in humans and animals. However, its role in health and production and the underlying mechanism in heat-stressed broilers are poorly understood. This study aimed to determine the effect of microalgae supplementation on the health and production of heat-stressed broilers. Cobb500 day-old chicks (N = 144) were raised in litter floor pens (6 pens/treatment and 8 birds/pen). The treatment groups were: 1) no heat stress (NHS), 2) heat stress (HS), and 3) heat stress + 3% microalgae (HS+MAG). The broilers in the HS+MAG group were fed a diet supplemented with 3% microalgae, whereas NHS and HS groups were fed a standard broiler diet. Broilers in the NHS were raised under standard temperature (20°C-24°C), while HS and HS+MAG broilers were subjected to cyclic heat stress from d 22 to 35 (32°C-33°C for 8 h). Heat stress significantly decreased the final body weight, whereas the supplementation of microalgae increased the final body weight of broilers (P < 0.05). The expressions of ileal antioxidant (GPX3), immune-related (IL4), and tight-junction (CLDN2) genes were increased in microalgae-supplemented broilers compared to heat-stressed broilers (P < 0.05). The ileal villus height to crypt depth ratio was improved in microalgae-supplemented broilers (P < 0.05). In addition, microbial alpha, and beta diversities were higher in the HS+MAG group compared to the HS group (P < 0.05). There was an increase in volatile fatty acid-producing bacteria at the genus level, such as Ruminococcus, Ocillospira, Lactobacillus, Oscillobacter, Flavonifractor, and Colidextribacter in the group that received microalgae supplementation. In conclusion, dietary supplementation of microalgae improved the growth performances of heat-stressed broilers by improving their physiogenomics. Thus, the dietary inclusion of microalgae can potentially mitigate heat stress in broilers.