Providing an optimal thermal environment is essential for efficient broiler production, particularly in tunnel-ventilated broiler houses where temperature and humidity distributions can vary significantly. However, despite the complexity of tunnel ventilation systems, the homogeneity of temperature distribution within the broiler house cannot be guaranteed. In particular, poorly planned tunnel ventilation systems can cause significant differences in the house temperature distribution, leading to excessive heat in some areas and cold places in others, negatively affecting the comfort and welfare of birds, and decreasing production efficiency. This study investigates the spatial variability of temperature and relative humidity in a tunnel-ventilated broiler house equipped with an evaporative cooling system during the summer. Measurements were taken at nine different points over a 42-day period, and analyses were conducted to assess the uniformity of the indoor climate. The results revealed significant differences in temperature and relative humidity between measurement points when the evaporative cooling system was in operation (p < 0.01). The mean relative deviation analysis indicated that thermal uniformity for temperature was 0.0240 during the daytime and 0.0169 at night, showing better thermal uniformity during nighttime conditions. Furthermore, Pearson correlation analysis revealed a strong relationship between cooling efficiency and spatial temperature variation (r = 0.656, p < 0.01), showing that increased airflow is related to these variations. Similarly, relative humidity variation was significantly correlated with cooling efficiency (r = 0.478, p < 0.01). These findings highlight that it is important to properly determine the air velocity and the location of air deflectors during the design phase to ensure a uniform environment in the house.