Canada's climate is warming faster than the global average, but the warming is unevenly distributed. This study analyzes historical and future climate change in dairy-producing regions across Canada to better understand how Canada's dairy cows are affected. Historical changes (i.e., 1960–2019) were assessed using temperature and humidity data from 29 weather stations across the country. The temperature–humidity index (THI) was used as an indicator of dairy cattle at risk of heat stress, and three THI metrics evaluated the frequency, severity, and duration of potential heat stress. Future scenarios were investigated using five global climate models to project daily THI under three Shared Socioeconomic Pathways (SSPs). Projections were grouped into three time periods (2020–2049, 2040–2069, and 2060–2089). Historical climate trends show an increase in temperature, humidity, and THI exceedance in most west coast and eastern Canada locations, affecting 84% of the national dairy herd. Future scenarios project that 90% of the national herd will experience a large increase in the frequency, severity, and duration of THI exceedance under all but the most optimistic SSP. These findings highlight the need for Canadian dairy farmers to consider heat-stress adaptation strategies.