We estimated the effects of Norway's universal ECEC program - expanding access to 1- and 2-year-olds starting in the early 2000s - on standardized math and achievement tests in 5th grade (age 10) using a population-based survey sample (Norwegian Mother, Father, and Child Cohort Study, MoBa, n = 102,352), linked with national administrative records of child achievement test scores. We used two methods to test the effects of attending ECEC: fixed-effects and instrumental variable regressions. Although both approaches found small and mostly insignificant main effects, the effect of ECEC is consistently stronger for children from families with low parental education. The fixed-effects analyses showed that ECEC reduced inequalities in achievement across levels of parental education by about 10% while the instrumental variable analyses, using variation in ECEC induced by the expansion, showed reduction of about 50%. These results suggest that expanding access to ECEC for toddlers has the potential to reduce achievement inequality.