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 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. (author abstract)