Promotion of adequate nutrition early in life through interventions aimed at improving infant and young child feeding (IYCF) has the potential to contribute to motor, cognitive, and social‐emotional development. This study evaluated this potential in Bangladesh where IYCF interventions were implemented in 300 sub‐districts through the Alive & Thrive (A&T) initiative.A cluster‐randomized, non‐blinded, impact evaluation design was employed comparing two intervention packages i.e., intensive interpersonal counseling on IYCF, mass media, and community mobilization vs. non‐intensive standard nutrition counseling and mass media. Twenty sub‐districts were randomized to either intensive or non‐intensive intervention areas. Cross‐sectional household surveys were conducted at baseline (2010) and four years later (2014) in the same communities. About 4,000 children aged 0–47.9 months in Bangladesh were surveyed at baseline and endline. Language development was measured with 21 items in which the mother was asked if her child had reached each milestone, ordered from least advanced to most advanced, resulting in a score from 0 to 21. Gross motor development was measured through mother report and observation with 29 ordered items, resulting in a score from 0 to 29. We derived difference‐in‐difference impact estimates using fixed‐effects regression models that assessed differences between the two intervention packages over time, adjusting for geographic clustering, infant age, gender, and covariates that differed between packages from baseline to endline.The difference‐in‐difference in language development scores between intensive and non‐intensive areas was 1.07 (p<0.001) among children 6–23.9 months and 0.78 (p<0.10) among children 24–47.9 months. For motor development, the difference‐in‐difference was 0.84 (p<0.10) among 6–23.9 month old children. From baseline to endline, the program had positive impact on language development among children 6–23.9 months (increase of 0.92 for intensive vs. decrease of 0.37 for non‐intensive) and a protective effect among children 24–47.9 months (increase of 0.07 for intensive vs. decrease of 0.87 for non‐intensive).The large‐scale intensive package in Bangladesh had substantial, significant impact on breastfeeding and complementary feeding practices (reported elsewhere). The intensive package, which promoted active, responsive feeding and strategies to manage poor appetite, had significant impact on language development and a marginally significant impact on motor development. Pathways to better development could include greater dietary diversity, more frequent feeding, and/or more effective strategies to manage poor appetite (all three could result in greater nutrient intake) and/or greater caregiver engagement with the child. Thus, both nutritional and behavioral mechanisms could be responsible for the improvements in child development; these mechanisms should be further studied.Support or Funding InformationFunding: Bill & Melinda Gates Foundation, through Alive & Thrive, managed by FHI 360; CGIAR Research Program on Agriculture for Nutrition and Health (A4NH), led by the International Food Policy Research Institute (IFPRI)