We examined whether older individuals with and without type 2 diabetes (T2D) experience differences in heart rate variability (HRV) during a 3-hour exposure to high heat stress compared to young adults. Young (YOUNG; n=22; 23±3 years) and older individuals with (T2D; n=11; 59±9 years) and without (OLDER; n=25; 63±5 years) T2D were exposed to heat stress (44°C, 30% relative humidity) for 3-hours. Fifty-five HRV measures were assessed for 15 minutes at baseline, and at minutes 82.5-97.5 (MID) and 165-180 (END) during heat stress. When compared to YOUNG, a similar number of HRV indices were significantly different (p<0.05) in OLDER (Baseline: 35; MID: 29; END: 32) and T2D (Baseline: 31; MID: 30; END: 27). In contrast, the number of HRV indices significantly different (p<0.05) between OLDER and T2D were far fewer (baseline: 13, MID: 1, END: 3). Within-group analyses demonstrated a greater change in the YOUNG group's HRV during heat stress, compared to OLDER and T2D; the number of significantly different (p<0.05) HRV indices between baseline and END were 42, 29, and 20, for YOUNG, OLDER, and T2D, respectively. Analysis of specific HRV domains suggest that YOUNG experienced greater sympathetic activity during heat stress compared to OLDER and T2D. In conclusion, compared to young, older individuals with and without type 2 diabetes demonstrate low HRV at baseline, and less change in HRV (including an attenuated sympathetic response) during 3-hour high heat stress, potentially contributing to impaired thermoregulatory function.