Abstract Background and Aims Children born after assisted reproductive technology (ART) have worse perinatal outcomes compared with spontaneously conceived children. This study investigates whether children conceived after ART have a higher risk of congenital heart defects (CHDs) compared with children born after spontaneous conception (SC). Methods All 7 747 637 liveborn children in Denmark (1994–2014), Finland (1990–2014), Norway (1984–2015), and Sweden (1987–2015), where 171 735 children were conceived after ART, were included. National ART and medical birth registry data were cross-linked with data from other health and population registries. Outcomes were major CHDs, severe CHDs, 6 hierarchical CHD lesion groups, and 10 selected major CHDs, diagnosed prenatally or up to 1 year of age (Denmark, Finland, and Sweden) and prenatally or at birth (Norway). The association between ART and CHDs was assessed with multivariable logistic regression analysis, with adjustment for available confounders. Results Major CHDs were detected in 3159 children born after ART (1.84%) and in 86 824 children born after SC [1.15%; adjusted odds ratio (AOR) 1.36; 95% confidence interval (CI) 1.31–1.41]. Risk was highest in multiples, regardless of conception method. Severe CHDs were detected in 594 children born after ART (0.35%) and in 19 375 children born after SC (0.26%; AOR 1.30; 95% CI 1.20–1.42). Risk was similar between ICSI and IVF and between frozen and fresh embryo transfer. Conclusions Assisted reproductive technology–conceived children have a higher prevalence of major CHDs, being rare, but severe conditions. The absolute risks are, however, modest and partly associated with multiple pregnancies, more prevalent in ART.