Global climate is changing and becoming warmer. Higher ambient temperatures and humidity increase stress and lower dairy cattle production, particularly in tropical countries. Temperature-humidity index (THI) has been used to assess the risk of heat stress in dairy cattle. When THI exceeds 72, cows are likely to begin experiencing heat stress. The objective of this research was to characterize yearly changes in THI and number of days that THI exceeded 72 (NHD) in Thailand from 2002 to 2014. Average daily temperatures and relative humidity were measured in 17 meteorological stations located in 5 regions across Thailand (n = 78,713). Regions were Northern, Northeastern, Central, Eastern, and Southern. The overall yearly means and SD were 81.9 and 4.2 for THI and 361.5 d and 13.8 d for NHD, and the corresponding ranges were 58.7 to 93.2 for THI and 166 to 366 d for NHD. The THI and NHD were analyzed using a model that included year, region and interaction between year and region as fixed effects, and residual as a random effect. All fixed effects influenced THI and NHD (P < 0.01). Yearly LSM ranged from 79.3 (Northeastern-2011) to 84.1 (Central-2010) for THI, and from 315 (Central-2002) to 366 d (Southern-2012) for NHD. Trends across years were non-significant for THI (b = −0.003 units/yr; P = 0.93) and for NHD (b = 0.39 d/yr; P = 0.34). However, larger fluctuations in THI existed from 2009 to 2014 than from 2002 to 2008 (81.3 to 83.3 vs. 81.5 to 82.4). Conversely, NDH showed similar ranges in both periods (349 to 364 d vs. 341 to 357 d). The LSM for THI and NHD were higher in the Central (83.0 and 354 d), Eastern (83.0 and 363 d), and Southern (82.8 and 361 d) regions than in the Northeastern (80.6 and 344 d) and Northern (80.6 and 346 d) regions. Although increasing trends in THI and NHD did not occur during the years of the study, variability in THI increased since 2009. Regional effects indicated that more comprehensive strategies for reduction of heat stress in dairy cattle might be needed in the Central, Eastern, and Southern Thailand than in the Northern and Northeastern parts of the country.