Abstract This paper investigates the impact of extreme heat on perinatal health disparities between migrant and native-born populations in Spain. Using birth records from 2018 to 2021, we analyze the effects of heat exposure during pregnancy, measured through orange-level heat alerts (> 37–40 °C), on birth outcomes such as absolute weight, the risk of low birth weight (< 2500 g), very low birth weight (< 1500), macrosomia (> 4000 g), and preterm birth (< 37th week). Our findings consistently reveal that migrant-origin infants face a higher risk of adverse outcomes compared with native-born infants, with the disparity being more pronounced in the first and second trimesters. The results challenge the notion that a “healthy immigrant effects” could be protective against the detrimental consequences of heat exposure. We conduct robustness checks using measures of absolute temperature exposures, confirming that the adverse effects of heat are consistent across different temperature measurements. These findings emphasize the urgent need to address heat-related health vulnerabilities, especially among migrant populations, as climate change intensifies.