Background: Temperature is considered one of the primary vital signs for detection of complications such as infections. Continuous wireless real-time axillary temperature monitoring is technologically feasible at the general ward, but no clinical validation studies exist. Methods: This study compared axillary temperature with a urinary bladder thermometer in 40 major abdominal postoperative patients. The primary outcome was changes in axillary temperature registrations. Secondary outcomes were mean bias between the urinary bladder and the axillary temperatures. Intermittent frontal and tympanic temperature recordings were also collected. Results: Forty patients were monitored for 50 min with an average core temperature of 36.8 °C. The mean bias was −1.0 °C (LoA −1.9 to −0) after 5 min, and −0.8 °C (LoA −1.6 to −0.1) after 10 min when comparing the axillary temperature with the urinary bladder temperature. After 20 min, the mean bias was −0.6 °C (LoA −1.3–0.1). During upper arm abduction, the axilla temperature was reduced to −1.6 °C (LoA −2.9 to −0.3) within 1 min. Temporal skin temperature measurement had a resulted in a mean bias of −0.1 °C (LOA −1.1 to −1.0) compared with central temperature. Compared with the mean tympanic temperature, it was −0.1 °C (LoA −0.9 to −1.0) lower than the urinay bladder temperature. Conclusions: Axillary temperature increased with time, reaching a mean bias of 1 °C between axillary and core temperature within 5 min. Opening the axillary resulted in rapidly lower temperature recordings. These findings may aid in use and designing corrections for continuous axillary temperature monitoring.