There is a need to rapidly screen individuals for heat strain and fever using skin temperature (Tsub.sk) as an index of deep body temperature (Tsub.b). This study’s aim was to assess whether Tsub.sk could serve as an accurate and valid index of Tsub.b during a simulated heatwave. Seven participants maintained a continuous schedule over 9-days, in 3-day parts; pre-/post-HW (25.4 °C), simulated-HW (35.4 °C). Contact thermistors measured Tsub.sk (Tsub.forehead, Tsub.finger); radio pills measured gastrointestinal temperature (Tsub.gi). Proximal-distal temperature gradients (ΔTsub.forehead–finger) were also measured. Measurements were grouped into ambient conditions: 22, 25, and 35 °C. Tsub.gi and Tsub.forehead only displayed a significant relationship in 22 °C (r: 0.591; p < 0.001) and 25 °C (r: 0.408; p < 0.001) conditions. A linear regression of all conditions identified Tsub.forehead and ΔTsub.forehead–finger as significant predictors of Tsub.gi (rsup.2: 0.588; F: 125.771; p < 0.001), producing a root mean square error of 0.26 °C. Additional residual analysis identified Tsub.forehead to be responsible for a plateau in Tsub.gi prediction above 37 °C. Contact Tsub.forehead was shown to be a statistically suitable indicator of Tsub.gi in non-HW conditions; however, an error of ~1 °C makes this physiologically redundant. The measurement of multiple sites may improve Tsub.b prediction, though it is still physiologically unsuitable, especially at higher ambient temperatures.