Transitional Kindergarten (TK) is a relatively recent entrant into the U.S. 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 3rd grade test scores. Using an augmented regression discontinuity design, we find that TK improves 3rd grade math scores by 0.29 standard deviations relative to a counterfactual that includes other formal and informal learning options. This impact is notably large relative to the prior pre-K literature. Estimates for English Language Arts (ELA) are imprecise but suggestive of a positive effect as well.