This is a multicenter, open-label extension study aimed to assess long-term safety and efficacy of SPN-812 ER when administered alone or in conjunction with an Food and Drug Administration-approved Attention Deficit Hyperactivity Disorder (ADHD) medication in the treatment of ADHD in pediatric subjects who have participated in a previous blinded study of SPN-812 ER. All pediatric subjects who complete a blinded study of SPN 812 ER will have the option to participate in this study in which all subjects receive SPN-812 ER. After an initial dose, subjects will enter an dose optimization phase for three to nine weeks. Following optimization, subjects will return to the clinic every 3 months or until the subject discontinues or the study ends at 36 months.