We examined the associations between preschool attendance and academic school readiness at kindergarten entry among 5-year-old children of immigrant mothers in the United States using data from a US nationally representative sample (Early Childhood Longitudinal Study-Birth Cohort, N = 1650). Comparing children who were in preschool (Head Start, prekindergarten, or other center-based preschool) to children being cared for exclusively at home, analyses using both ordinary least squares regressions with rich controls and with propensity score weighting consistently showed that attending preschool was associated with higher reading and math skills. Analyses focused on specific type of preschool revealed that children attending prekindergarten (but not Head Start and other center-based preschool) had higher reading and math skills than those in parental care. Analyses focused on hours of preschool attendance indicated that children's reading skills benefited from attending more than 20 hours per week of Head Start or prekindergarten. Attending preschool, especially for full days, increases the school readiness of children of immigrants.