The slick hair phenotype has been suggested as an alternative for dairy production in tropical and subtropical countries. The present study evaluated the relationship between hair coat type (phenotypically determined), vaginal temperature (VT) and thermal humidity index (THI) in lactating Puerto Rican Holstein cows. In trial 1, 4 wild type-haired (WT; 169.8 ± 3.8 d in milk (DIM) and 3.8 ± 2.2 lactations) and 5 slick-haired (SLICK; 146.6 ± 63 DIM and 3.0 ± 2.1 lactations) cows were used. In trial 2, 5 WT (173.2 ± 42.3 DIM and 3.0 ± 2.1 lactations) and 5 SLICK (163.8 ± 43.0 DIM and 2.8 ± 1.7 lactations) cows were evaluated. Data loggers recorded VT (TidbiT v2), air temperature, and relative humidity (HOBO Pro v2) every 5 min for 8 consecutive days and the THI was determined. The THI presented daily ranges from 63.72 to 80.63 and 61.79–81.47 on trial 1 and trial 2, respectively. Data were analyzed by the GLIMMIX and CORR procedures of SAS. In trial 1 there were no differences in VT between SLICK and WT cows (38.71 ± 0.08 and 38.71 ± 0.09°C, respectively; P = 0.96). Daily VT in SLICK and WT cows began to increase (P < 0.001) at similar THI values (68.49 and 65.02, respectively; P = 0.82). In this first trial, correlation coefficients of 0.27 (P < 0.0001) and 0.36 (P < 0.0001) were found between the VT and THI in SLICK and WT cows, respectively. In trial 2, hair coat type and time of the day interacted to affect VT (P < 0.001). During the 1900–2100 h WT cows presented VT values 0.25°C greater than SLICK cows (P = 0.02). From 2200 to 1800 h WT and SLICK cows presented similar VT values (38.68 ± 0.10 and 38.60 ± 0.07°C, respectively; P = 0.39). Also in trial 2, daily VT began to increase (P < 0.001) in both hair type groups when THI reached 70.08. In this second trial, correlation coefficients of 0.32 (P < 0.001) and 0.42 (P < 0.001) were found between the VT and THI in SLICK and WT cows, respectively. In general, environmental conditions had a greater association with the vaginal temperature in wild type cows than in their slick-haired counterparts, and in trial 2 than in trial 1. However, vaginal temperature only differed between hair type groups during a limited period.