Objectives Over-nutrition during pregnancy resulting from maternal obesity or an unhealthy diet can lead to excess infant adiposity at birth. Specific dietary macro- and micronutrients have been shown to increase fat cell development in both in-vitro and in-vivo models and may therefore link maternal diet to increased infant adiposity. We hypothesized that high maternal dietary niacin intake during pregnancy, especially in combination with a high-fat diet (HFD) would increase infant adiposity. Methods We included 1040 participants from a pre-birth cohort of mother-infant pairs. Maternal diet was assessed using multiple 24-hour dietary recalls. HFD was defined as ≥30% of calories from fat and ≥12% of fat calories from saturated fat. Neonatal body composition (% fat mass [%FM], fat mass [FM], fat-free mass [FFM]) was measured by PEAPOD. We used multivariate regression to assess the joint effect of maternal dietary niacin and maternal HFD on neonatal body composition. Results Dietary niacin was not associated with neonatal body composition, and maternal HFD did not modify this finding. However, maternal HFD was independently associated with %FM (β = 0.8 [0.1, 1.4]%, p < 0.01] and FM (β = 32.4 [6.7, 58.0] g, p < 0.01). Conclusions for Practice Our results suggest that a HFD during pregnancy may increase infant adiposity, therefore supporting the need for improved diet counseling of pregnant women at both the clinical and community levels.