Four consecutive studies aimed to test the validity of an implantable passive radio frequency identification (RFID) microchip (Bio-thermo, Allflex USA) in various implant sites in a calf as an alternative to rectal temperature (RT) measurements. Preimplantation, microchips were validated in a water bath against a rectal thermometer, and we found a very high correlation to the rectal thermometer (r > 0.95) with no measurement bias found in Bland-Altman plots. Three microchips were implanted per calf: subcutaneously behind the ear (EAR), subcutaneously by the upper scapula (SCAP), and intramuscularly in the neck (NECK). Microchips were tested for repeatability of measurements when implanted; each calf (n = 11) had an average of 24 observations per site all taken within 2 min. The average coefficient of variance for microchips were low (0.12 ± 0.03% CV) and the difference between measurements were negligible (0.1 ± 0.04°C, range 0–0.4°C). Following, calves (n = 12) were enrolled in a 24-h study, where each calf had their RT, microchips, and tympanic temperature measured hourly. Correlations were high between SCAP vs NECK (individual animal correlation; median [Q1, Q3] r = 0.75 [0.60, 0.84]; P ≤ 0.02) and EAR vs NECK (r = 0.78 [0.73, 0.84]; P ≤ 0.01). From the 24 h RT had negligible correlations with the 3 microchips sites and tympanic temperature, RT’s highest correlation was with tympanic (r = 0.19 [0.07, 0.22]; P ≤ 0.81). Finally, calves (n = 10) were enrolled in a long-term study where daily for 30d RT and the microchips were measured. Rectal temperature had negligible correlations with EAR and NECK, and RT had a low correlation with SCAP (r = 0.33; P < 0.001). Also, correlations were high for EAR vs NECK (r = 0.79 [0.73, 0.89]; P ≤ 0.02); the weakest correlation was between EAR vs RT (median r = 0.06 [0.03, 0.14]; P ≤ 0.99). Our results suggest that microchip temperature readings are consistent among microchips, but readings vary by implant site and has a weak linear relationship with RT. Microchips are promising technologies, but further testing is necessary to support the use to determine body temperature and its deviations in dairy calves.