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Abstract Measurement of body core temperature (Tcore) is paramount to determining the thermoregulatory strain of athletes. However, standard measurement procedures of Tcore are not practical for extended use outside the laboratory environment. Therefore, determining the factors that predict Tcore during a self‐paced run is crucial for creating more effective strategies to minimize the heat‐induced impairment of endurance performance and reduce the occurrence of exertional heatstroke. The aim of this study was to identify the factors predicting Tcore values attained at the end of a 10 km time trial (end‐Tcore) under environmental heat stress. Initially, we extracted data obtained from 75 recordings of recreationally trained men and women. Next, we ran hierarchical multiple linear regression analyses to understand the predictive power of the following variables: wet‐bulb globe temperature, average running speed, initial Tcore, body mass, differences between Tcore and skin temperature (Tskin), sweat rate, maximal oxygen uptake, heart rate and change in body mass. Our data indicated that Tcore increased continuously during exercise, attaining 39.6 ± 0.5°C (mean ± SD) after 53.9 ± 7.5 min of treadmill running. This end‐Tcore value was primarily predicted by heart rate, sweat rate, differences between Tcore and Tskin, wet‐bulb globe temperature, initial Tcore, running speed and maximal oxygen uptake, in this order of importance (β power values corresponded to 0.462, −0.395, 0.393, 0.327, 0.277, 0.244 and 0.228, respectively). In conclusion, several factors predict Tcore in athletes subjected to self‐paced running under environmental heat stress. Moreover, considering the conditions investigated, heart rate and sweat rate, two practical (non‐invasive) variables, have the highest predictive power.