Chronic cyclic heat stress impairs thermoregulation, growth, tissue integrity, microbial metabolism, and stress-related molecular responses in broiler chickens. N-acetyl-L-cysteine, a cysteine donor involved in glutathione synthesis, has been proposed as a nutritional strategy to support stress adaptation. Therefore, this study evaluated NAC in 324 Ross 308 broilers allocated to thermoneutral conditions (TN, 7 pens), HS (HS, 10 pens), or HS plus 2000 mg/kg NAC (HS + NAC, 10 pens). From day 20 to 35, HS birds were exposed to 34 °C for 7 h/day and 26 °C otherwise, while TN birds were kept at 22 °C. Heat stress increased rectal temperature and respiratory rate (p < 0.001), reduced average daily gain, and increased drinking and gasping behavior (p < 0.05). Heat stress also increased hepatic cord disorganization, cerebral vacuolisation–necrocytosis, hepatic and cerebral HSP70 expression, and caecal propionate, while reducing cerebral apelin expression (p < 0.05). Jejunal villus morphology and microbiota α-diversity were unchanged, although caecal profiles indicated treatment-related shifts and potentially higher Bilophila spp. in heat-stressed birds. NAC did not improve growth, thermoregulation, oxidative status, caecal fermentation, microbial structure, or HSP70 expression. It increased jejunal villus fusion and hepatocellular vacuolation scores and reduced cerebral ghrelin expression (p < 0.05). In conclusion, heat stress impaired growth and induced tissue, microbial, fermentative, and cerebral gene-expression changes, whereas 2000 mg/kg NAC did not provide measurable protection under the chronic cyclic HS in the present study.