We estimate the effect of introducing universal full-day Kindergarten (FDK) on subsequent Grade 4 test scores. The data are from the Canadian province of British Columbia, which moved from mostly half-day to universal full-day Kindergarten between the 2010/11 and 2011/12 academic years. We exploit the staggered timing of the policy implementation using a difference-in-differences research design. Our point estimates for the average effect of FDK on achievement are mostly positive, occasionally statistically significant, and always small. The effect is substantially larger among students who speak English as a second language, a result that is consistent with prior findings.