<p>Record-high temperatures and more frequent heatwaves are well-documented consequences of climate change, but the developmental consequences of heat during sensitive periods such as pregnancy and early childhood are unclear. We conducted a pre-registered systematic review (PROSPERO CRD42024596655) of empirical studies examining associations of ambient heat exposure during the prenatal period or ages 0–5 with developmental outcomes measured later in childhood, adolescence, or adulthood. Eleven databases were searched in August 2024 and supplemented with backward and forward citation tracking. Two reviewers independently screened, extracted, and appraised studies using the Mixed Methods Appraisal Tool. Findings were synthesized narratively due to heterogeneity in design, exposure, and outcomes. Of 11,955 records, five studies met inclusion criteria. Four reported that higher early-life heat exposure was associated with adverse cognitive, mental health, or behavioural outcomes across the life course, while one found null effects. Effect modification was observed by developmental timing, region, season, and contextual factors. Retrieved studies were geographically narrow and methodologically heterogeneous. Although data are limited, emerging findings suggest that heat exposure in early life may have lasting adverse effects on development. More rigorous and more globally representative studies are needed to clarify mechanisms, sensitive windows, and moderators, and to inform prevention and adaptation.</p>