Adolescents with externalizing behaviors are at risk for multiple social and health impairments that may persist into adulthood. Although empirically supported treatments are available for teenagers with severe externalizing behavior and their families, only a few parenting programs have been evaluated for adolescents with subclinical levels of behavior problems. In the present randomized controlled trial (N = 75), we examined the feasibility and efficacy of the Parent-Web, a new Internet-based parenting program for parents of teenagers. Participants reported high satisfaction with the program, 74% completed all five modules and dropout was only 14%. Generalized linear mixed models showed short-term (8 weeks) improvements in parent-reported teenager externalizing behaviors, family conflicts, and family warmth compared to a waitlist control condition. Receiving the Parent-Web was also associated with more improvements in parent-reported teenage emotional symptoms and peer problems, as well as parental stress and depression. No differences regarding parenting behaviors between the two groups were found. Those in the intervention group maintained their posttreatment gains on primary outcomes at the follow-up assessment, 6- to 9-months after the program. Although limited by relying solely on parent report, and by a relatively small sample size, we conclude that our study contributes some preliminary evidence for the utility of supporting parents of adolescents at risk of developing behavior problems through an Internet-based intervention. (PsycINFO Database Record (c) 2019 APA, all rights reserved).