Transitional kindergarten (TK) is a relatively recent entrant into the US early education landscape, combining features of public pre-K and regular kindergarten. We provide the first estimates of the impact of Michigan's TK program on third-grade test scores. Using an augmented regression discontinuity design, we find that TK improves third-grade test scores by 0.29 (math) and 0.19 (English language arts) standard deviations relative to a counterfactual that includes other formal and informal early learning options. These impacts are notably large relative to the prior pre-K literature.