Purpose: The purpose of this randomized controlled trial was to examine the effects of a multitiered system of language support (MTSLS) on kindergarteners' narrative retelling, personal stories, writing, and expository language. Method: Participants were 686 kindergarten students from four school districts in the United States. Twenty-eight classrooms were randomly assigned to treatment (n = 337 students) or control (n = 349 students) conditions. The treatment group received 14 weeks of oral narrative language instruction using "Story Champs," a multitiered language program. Classroom teachers delivered large group (Tier 1) instruction for 15-20 min a day for 4 weeks. After this short-duration whole-class instruction, speech-language pathologists began small group "Story Champs" (Tier 2) intervention with a random sample of students who did not make adequate progress from the large group instruction (n = 49). These students received Tier 2 intervention for 20 min twice a week in addition to continued Tier 1 instruction. Results: Results indicated that the students in the treatment group had significantly higher scores on all outcome measures compared to the students in the control group. Analyses of outcomes from the 49 students who received Tier 2 intervention compared to a matching sample of at-risk control students revealed that the treatment group had significantly higher scores on narrative retells, personal stories, and expository retells. When compared to matched average-performing and advanced-performing control peers, the students who received Tier 2 intervention had significantly higher narrative retell scores and no longer had significantly lower personal story, expository, or writing scores. Conclusion: This effectiveness study demonstrated that MTSLS can lead to meaningful improvements in kindergarteners' oral and written language skills, even helping at-risk students catch up to high-achieving peers.