Background: Cognitive development in early childhood is critical for life-long well-being. Existing cognitive development surveillance tools require lengthy parental interviews and observations of children. Developmental Assessment on an E-Platform (DEEP) is a digital tool designed to address this gap by providing a gamified, direct assessment of cognition in young children which can be delivered by front-line providers in community settings. Methods: This longitudinal study recruited children from the SPRING trial in rural Haryana, India. DEEP was administered at 39 (SD 1; N=1359), 60 (SD 5; N=1234) and 95 (SD 4; N=600) months and scores were derived using item response theory. Criterion validity was examined by correlating DEEP-score with age, Bayley’s Scales of Infant Development (BSID-III) cognitive domain score at age 3 and Raven’s Coloured Progressive Matrices (CPM) at age 8; predictive validity was examined by correlating DEEP-scores at preschool-age with academic performance at age 8 and convergent validity through correlations with height-for-age z-scores (HAZ) and early life adversities. Findings: DEEP-score correlated strongly with age (r=0.83, 95% CI 0.82-0.84) and moderately with BSID-III (r=0.50, 0.39-0.60) and CPM (r=0.37; 0.30 – 0.44). DEEP-score at preschool-age predicted academic outcomes at school-age (0.32; 0.25 – 0.41) and correlated positively with HAZ and negatively with early life adversities. Interpretation: DEEP provides a valid, scalable method for cognitive assessment. It’s integration into developmental surveillance programs could aid in monitoring and early detection of cognitive delays, enabling timely interventions. Funding: SPRING, REACH and COINCIDE were funded through Wellcome Trust, Madura Microfinance Ltd and Wellcome Trust/DBT India Alliance respectively.