Background: Dairy cattle in many tropical, subtropical and semi-arid regions are subject to high ambient temperature and relative humidity for extended periods of time. This compromises the ability of the lactating cows to dissipate heat, resulting in heat stress and decreasing milk production. The objective of this study was to evaluate the effect of the temperature-humidity index (THI) on milk production in cows in the study area. Methods: Data on monthly milk production (2017 to 2019) was collected from the Department of Animal Production and Health, Batticaloa District, Sri Lanka and the monthly weather data (2017 to 2019) required for the study was collected from the Meteorological Observatory Station in Batticaloa, Sri Lanka. SPSS version 26 was used to analyze interaction between milk production and weather parameters. The monthly THI values were determined for each year.Result: Statistically significant negative relationship (p less than 0.05) was found between milk production and THI in 2017, 2018 and from 2017 to 2019 except 2019 due to the sudden death of cattle in Batticaloa district. Results of all three years indicated that milk production decreases as THI increases. It is concluded that the milk yield of cows is influenced significantly by heat stress during the dry season from April to August between 2017 and 2019, proper management strategies could be helpful to minimize heat stress to attain optimal performance of dairy cows.