This study aimed to estimate the effect of temperature-humidity index (THI) and the magnitude of genotype-by-THI (GxTHI) interactions on milk production traits and on somatic cell score in Montbéliarde cows. Test-day records from first and second lactations from 2016 to 2020 were associated with the average THI of the three days before the test-day record. A total of 301,078 test-day records from 39,469 cows in first lactation and 292,648 test-day records from 37,593 cows in second lactation were analyzed. The average THI effect, estimated with an animal mixed model, indicated that the optimal THI was below 55 (ie ~ 12-13°C). Individual responses to THI were estimated simultaneously with the time-dependent response by random regression models. Regardless of the stage of lactation, genetic correlations along the THI gradient were > 0.80 and most of them were > 0.90, suggesting that GxTHI interactions were weak for production and for udder health traits.