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Heat stress (HS) affects dairy cow fertility and can lead to substantial economic losses. This study aimed to examine the effect of HS on the mRNA expression of selected genes in the corpus luteum (CL). The study was conducted on commercial farms in Slovakia and Argentina. Animals (n = 86) were included on the day of artificial insemination (AI) and examined on days 7, 14 and 21 post AI. The rectal temperature (RT) was measured weekly and CL tissue samples were collected on day 21 post AI by transvaginal biopsy. Pregnancy checks were done on day 42 post AI. Climate data were collected every 15 min using data loggers. HS exposure was assessed by calculating the area under the curve between the temperature humidity index (THI) threshold of 68 and the THI amplitude. The CL tissue was used for RT-qPCR for selected genes (FGF1, IL1B, IL10, ISG15, MX2, OAS1, PTGS2, VEGF2, HSP70, HSP90B2, STAR, TNF, and VEGFC). During the study period, 62% of the animals experienced HS, and a positive correlation was found between RT and THI values on days 7, 14 and 21 post AI (p < 0.05). The pregnancy rate was lower in HS cows (15.1%) than in non-HS cows (55.6%). The mRNA expression of IL1B, ISG15, OAS1, VEGF2, HSP70, and VEGFC significantly differed between pregnant and nonpregnant animals (p < 0.05). In pregnant animals, IL1B was lower and ISG15 was greater expressed in HS cows compared to no-HS cows (p < 0.05). The results illustrate that pregnancy status and HS significantly affect mRNA expression in the CL. Further work is required to understand the complex associations between HS, CL function and fertility.