Hair sheep dissipate heat mainly through respiratory evaporation, making panting an important physiological response to heat stress. This study evaluated the panting score (PS) as an indicator of heat stress in field-managed hair sheep. Fifty-two non-pregnant, black-coated Santa Inês ewes were kept in paddocks without shade for five consecutive days from 07:00 to 18:00. Respiratory rate (RR) was recorded every 30 min, vaginal temperature (T_VG) was continuously monitored, and PS was assessed every two hours using a 0–4 scale. Meteorological variables were recorded, and the temperature–humidity index (THI) was calculated. Associations were analyzed using Pearson’s correlation, mixed (T_VG), and generalized additive (RR) models. Animals were exposed to severe and, at times, extreme heat stress conditions (THI ≥ 25.6) between 10:00 and 16:00. PS showed a strong correlation with RR (r = 0.74) and moderate associations with air temperature (r = 0.69), THI (r = 0.49), and solar radiation (r = 0.47). Higher PS values (2–3) predominated during peak solar radiation. Increases in PS were associated with elevations in RR and T_VG (~0.66 °C). Broken-line regression identified a THI inflection point at 24.44, above which the T_VG increase was attenuated by 46%. These results establish PS as a valid field indicator of heat stress in hair sheep and provide a THI inflection point (24.44) above which thermoregulatory capacity is engaged, enabling more accurate on-farm heat stress monitoring in tropical extensive systems in the future.