In a previous pivotal study of children and adolescents (aged 6-17 years) with attention-deficit/hyperactivity disorder (ADHD), dose-optimized SHP465 mixed amphetamine salts (MAS) extended-release (12.5-25 mg once daily) was superior to placebo in reducing ADHD symptoms. This study evaluated the efficacy, tolerability, and safety of 6.25 mg SHP465 MAS once daily (one-half the lowest approved dose for adolescents and adults) versus placebo in children aged 6-12 years with ADHD. Children (aged 6-12 years) with Diagnostic and Statistical Manual of Mental Disorders, Fifth edition-defined ADHD; baseline ADHD-Rating Scale, Fifth Edition, Child, Home Version total scores (ADHD-RS-5-HV-TS) ≥28; and baseline Clinical Global Impressions-Severity scores ≥4 were eligible. Participants received 6.25 mg SHP465 MAS once daily or placebo for 4 weeks. The primary (ADHD-RS-5-HV-TS change from baseline at week 4) and key secondary (Clinical Global Impressions-Improvement [CGI-I] score at week 4) efficacy end points were assessed using linear mixed-effects models for repeated measures. Safety and tolerability assessments included treatment-emergent adverse events (TEAEs) and vital sign changes. Of 89 randomized participants, 83 completed the study (placebo,  = 41; SHP465 MAS,  = 42). At week 4, the least squares mean (95% confidence interval) treatment differences (SHP465 MAS-placebo) were not statistically significant for ADHD-RS-5-HV-TS change (-1.9 [-6.8 to 3.1],  = 0.451; effect size [ES] = 0.17) or CGI-I score (-0.1 [-0.5 to 0.3], nominal  = 0.597; ES = 0.12). The percentage of participants reporting TEAEs was 16.3% with placebo and 24.4% with SHP465 MAS. The most frequently reported TEAEs (placebo; SHP465 MAS) were headache (7.0%; 4.4%) and decreased appetite (4.7%; 2.2%). Minimal increases in blood pressure were observed with SHP465 MAS at the final on-treatment assessment. SHP465 MAS 6.25 mg once daily (one-half the lowest dose approved for adolescents and adults) was well tolerated in children aged 6-12 years but was not superior to placebo in reducing ADHD symptoms, suggesting that this dose of SHP465 MAS was subtherapeutic in this age group. The Clinical Trial Registration number: NCT03325881.