This is a randomized, double-blind, placebo-controlled, multicenter, 2-arm (1:1), parallel group, efficacy, and safety/tolerability fixed-dose study of SPN-812 in preschool-age children (4 to 5 years old) with a DSM-IV-TR diagnosis of ADHD. Approximately 286 subjects will be randomized to either SPN-812 or matching placebo in a 1:1 ratio (143 subjects per arm). Following up to 4 weeks of screening, subjects will be treated with study medication (SM) for 6 weeks, then will either be enrolled in a separate OLE study or followed for an additional 1 week for safety. The total duration of the study is up to 10 weeks.