The current study evaluated the role of dietary fennel essential oil (FO) supplementation in ameliorating the effects of heat stress on growth performance, meat quality, antioxidant activity, inflammatory responses, and liver histopathology in broiler chickens. Six hundred male broiler chicks (three-day-old ROSS broilers) were allocated into six experimental treatments (TRTs); the first was the negative control (Neg. CON, not subjected to hot temperature conditions), and the second was the positive control group (PS CON, subjected to hot temperatures). The control groups (Neg. and PS) were fed the basal diet without supplements. The third, fourth, and fifth groups were fed diets supplemented with 1 g FO/kg diet, 2 g FO/kg diet, and 3 g FO/kg diet, respectively and subjected to hot temperatures. The sixth group was fed the basal diet, received 500 mg of paracetamol per liter of drinking water, and subjected to hot temperatures. Hot temperature conditions (36 ± 0.5 ◦C) was applied for 6 h/day from the 22nd to the 25th day of the feeding period. The feeding period lasted for 35 days. The results showed that FO supplementation improved the body weight, weight gain, and feed conversion ratio compared to those in the PS CON and paracetamol groups. The growth hormone concentrations increased in the FO-supplemented TRTs and the paracetamol groups compared to those in the Neg. and PS CON groups. The serum total protein, albumin, and globulin concentrations significantly increased in the FO-supplemented TRTs compared to those in the Neg. and PS CON groups and the paracetamol TRTs. The serum TAC increased in the 3 g FO/kg TRT. The serum activity of CAT and SOD increased in the 3 and 2 g FO/kg TRTs and the paracetamol TRTs compared to those in the Neg. and PS CON groups. The serum MDA concentrations decreased in the FO-supplemented TRTs and paracetamol groups compared to those in the Neg. and PS CON groups. The IL1β and IFN-α concentrations decreased in the FO-supplemented and paracetamol groups compared to those in the PS CON groups. The HSP70 concentration was the highest in the 3 g FO/kg TRT. The immune expression of IL1-β and TGF-β in the liver tissues was downregulated in the FO-supplemented groups, especially the FO3 group, compared to those in the PS CON group. In conclusion, dietary supplementation with FO increased the broiler chickens’ growth more than that in the PS CON and paracetamol groups under hot temperatures. Fennel oil supplementation (3 g/kg diet) can alleviate the negative impacts of heat stress on broiler chickens’ antioxidant and inflammatory responses.