This study investigated the effect of the digital Down Syndrome LanguagePlus (DSL+) intervention on vocabulary outcomes through a school-delivered randomized controlled trial. A national sample of first graders with Down syndrome from 91 schools was allocated to an intervention group (n = 50), which received daily intervention for 15 weeks, or a business-as-usual control group (n = 53). The intervention involved picture book sharing and structured tasks and was organized as one-to-one, small-group, and full-class lessons. Children in the intervention group made greater gains than children in the control group in expressive vocabulary breadth (d = 0.429, CI [0.160, 0.699]) and receptive vocabulary breadth (d = 0.447, CI [0.193, 0.700]). The outcomes indicate that the novel DSL+ intervention is an effective intervention to increase trained vocabulary among first graders with Down syndrome, and it takes only 15 minutes of effort 5 days a week.