Recent literature on parent-child sex communication (PCSC) has identified optimal sex communication as being not only frequent, but also open in quality, and proactive in timing. However, no studies have sought to understand what motivates parents to participate in this type of optimal parent-child sex communication, nor assessed these relationships longitudinally. Using Self-Determination Theory (SDT) to inform the analysis, this study investigated parent characteristics (parent's sexual self-concept and parenting dimensions) that may longitudinally predict optimal PCSC. This study further examined whether parental sex communication efficacy mediated these relationships. The sample included 607 mother-child dyads, and the father-figure when available (mother-father-child triads (n = 363); mother-child dyads (n = 244)). Adolescent children were 12-17 years old (Wave 1 Mage = 14.55, SD = 1.68), White (56%), and cisgender female (51%). Data were collected annually for 3 years. A longitudinal SEM analysis was utilized. Results revealed that parental sex communication efficacy is longitudinally associated with children's reports of optimal sex communication. The discussion will focus on the importance of efficacy in improving parent-child sex communication processes.