Background: Diet in the first 2 y of life may be a pivotal period regarding effects on future blood pressure (BP). However, data on early-life diet and BP in childhood are sparse. Objective: We prospectively assessed associations between types of diet spanning infancy and toddlerhood (ie, transition diets across the complementary feeding period) and BP at age 7.5 y. Design: In a birth cohort study (Avon Longitudinal Study of Parents and Children; United Kingdom), a total of 1229 children had complete dietary intake data at 6, 15, and 24 mo; BP data at 7.5 y of age; and all 18 covariables. Results: Of the 2 transition diets that were extracted by using principal components analysis, the less-healthy diet was associated with an increase in systolic BP of 0.62 mm Hg (95% CI: 0.00, 1.24 mm Hg) and an increase in diastolic BP of 0.55 mm Hg (95% CI: 0.10, 1.00 mm Hg) for every one-unit (SD) increase in the less-healthy-diet score after adjustment for 15 potential confounders, including maternal characteristics and sociodemographic factors, birth variables, and breastfeeding duration. In contrast with systolic BP, the positive association between the less-healthy transition-diet score and diastolic BP persisted after additional adjustment for child body-size factors [height, body mass index (BMI), and waist circumference] at 7.5 y. Conclusions: A less-healthy transition diet by age 2 y was associated with higher BP at 7.5 y. The BMI-related reduction in effect size reinforces the importance of BMI on the diet-BP relation.