This study examined the effects of a dietary synbiotic supplement on the behavioral patterns and growth performance of broiler chickens exposed to heat stress (HS). Three hundred sixty 1-day-old male Ross 708 broiler chicks were distributed among 24 floor pens (15 chicks per pen); each pen was randomly assigned to one of 3 dietary treatments containing a synbiotic at 0 (control), 0.5 (0.5X) and 1.0 (1.0X) g/kg. From d 15 to 42, birds were exposed to HS at 32°C daily from 08:00 to 17:00. Five broiler chickens were randomly marked in each pen for behavioral observation. Instantaneous scan sampling was used to record the birds' behavioral patterns. Performance parameters were measured on d 7, 14, 28 and 42. The synbiotic fed birds exhibited more standing, sitting, walking, feeding, preening and less wing spreading and panting behaviors (P < 0.05) compared to birds fed the control diet. The synbiotic group also had higher BW, BW gain and feed intake on d 7, 14 and 42 (P < 0.05), and higher BW, feed intake and feed conversion ratio at d 28 (P < 0.01). There were no treatment effects on drinking behavior, BW gain on d 28 and feed conversion ratio on d 42 (P > 0.05). There were few dose-related differences of the synbiotic on production performance; namely, the 1.0X concentration resulted in the highest BW and feed intake on d 14 and 42 (P < 0.05), while BW gain was higher compared to the control group only on d 42 (P < 0.05). The results suggest that the synbiotic supplement may prove to be an important management tool for the broiler industry to diminish the negative effects of HS, potentially safeguarding the welfare and production of broiler chickens, particularly in areas that experience hot climates.