Many states and communities have invested in public early childhood education programs to improve children's readiness to enter school and narrow achievement gaps in later grades. This study asked whether and how Wisconsin's universal state-funded prekindergarten program, Wisconsin 4K, has improved student achievement and helped to reduce the achievement gap. Using publicly available data from 2002-2003 to 2013-2014, the study examined the effects of the program, which features high participation rates and part-day delivery modes. The results showed that Wisconsin 4K enhanced third-grade reading achievement but not math achievement in the participating districts. Its effects were larger for non-White and economically disadvantaged students than for their White and affluent counterparts. The policy implications for large-scale universal pre-K are discussed.