The temperature differential (ΔT) between a body surface and the environment influences an organism's heat balance. In Sarasota Bay, FL, where ambient water temperature (Tw) ranges annually from 11° to 33°C, ΔT was investigated in a resident community of bottlenose dolphins (Tursiops truncatus). Dorsal fin surface temperatures (Tdfin) were measured on wild, free-swimming dolphins using infrared thermography. Field and laboratory calibration studies were also undertaken to assess the efficacy of this non-invasive technology in the marine environment. The portability of infrared thermography permitted measurements of Tdfin across the entire range of environmental temperatures experienced by animals in this region. Results indicated a positive, linear relationship between Tdfin and Tw (r2= 0.978, P < 0.001). On average, Tdfin was 0.9°C warmer than Tw across seasons, despite the 22°C annual range in Tw. Changes in integumentary and vascular insulation likely account for the stability of ΔTdfin − w and the protection of core temperature (Tcore) across seasons. The high thermal conductivity of water may also influence this ΔT. The use of infrared thermography is an effective, non-invasive method of assessing dorsal fin skin surface temperatures (±1°C) across large numbers of wild, free-swimming dolphins throughout their thermally dynamic aquatic environment.