Background Increasing evidence suggests that climate change, along with its cascading impacts on ecosystems, societies, and communities, has significant effects on both physical and mental health. However, less is known about how exposure to excessive heat early in life may influence the development of foundational skills that shape lifelong developmental trajectories. This study examined the effects of ambient heat on early childhood development across six countries, using geographic and time‐stamped data on child development and ambient temperature. Methods Our primary outcome is the Early Childhood Development Index. We used linear probability models with geographic and seasonality fixed effects to account for baseline climatic conditions, as well as other individual and contextual covariates to address potential selection bias. The sample comprised 19,607 children aged three and four from Georgia, The Gambia, Madagascar, Malawi, Sierra Leone, and the State of Palestine, all participants in Multiple Indicators Cluster Surveys collected between 2017 and 2020. We merged these data with temperature data from the ERA5‐Land Monthly Aggregated Climate Dataset, calculating the mean monthly maximum temperature children experienced from birth to interview. Results We found that children exposed to average maximum temperatures above 32°C were less likely to be developmentally on track compared to those exposed to cooler temperatures, even after accounting for baseline average climatic conditions and other covariates. Domain‐specific models indicate that these effects were most pronounced in literacy and numeracy skills. Subgroup analyses revealed that the negative impacts were particularly severe for children in economically disadvantaged households and urban areas, and for those lacking access to adequate water and sanitation. Conclusions This study highlights the potential impact of excessive heat on early childhood development, emphasizing the need for policies and interventions that enhance preparedness, adaptation, and resilience to support human development in an rapidly warming world.