Coaches are instrumental in creating safe sport environments, especially in preventing sexual violence, but little is known about helpful bystander behaviors, hampering effective prevention programs. To identify determining characteristics of positive bystander behavior, 1442 Belgian youth-sport coaches completed a dedicated online questionnaire on bystander-related attitudes, descriptive and injunctive norms, autonomy beliefs, and self-efficacy using two hypothetical sport-associated sexual-violence scenarios. Potential for change was analyzed using confidence interval-based estimation of relevance (CIBER). 127 coaches (9.6%) had witnessed sexual violence over the past year. Most had intervened (single incident: 3.7%; multiple incidents: 2.4%). Experiential attitude expectation, instrumental attitude evaluation, perceived referent behavior and approval, and subskill presence were positively associated with coaches’ intentions to intervene. Of the determinants of positive coach-bystander behavior, attitude and perceived norms proved key constituents for programs addressing sexual violence in youth sport. To promote (pro-)active coach-bystander behaviors, the results are discussed from a theoretical and practice-oriented perspective.