Latino grandparents are often involved in rearing grandchildren, but their roles in promoting children's physical activity (PA) have been largely overlooked. This study examined the impact of a community-based intervention called Nietos Activos y Saludables (Active and Healthy Grandchildren) on Latino grandparents' knowledge about and self-efficacy in supporting grandchildren's (aged 2-12 years) PA.The Spanish language intervention, consisting of 4 weekly sessions, was delivered to 12 Latino grandparents (primarily female, aged > 60 years) in Los Angeles County from October to November 2017. Pre- and posttest surveys and postintervention focus groups assessed the intervention impact.Participants indicated that the intervention provided useful information. After intervention sessions, participants achieved a higher score in test on PA requirements for children. They also reported greater knowledge about children's PA, local resources for children's PA, and modifying children's behavior. Additionally, participants reported greater confidence in motivating children in PA and collaborating with parents in promoting children's PA. Results of focus groups generally confirmed the quantitative findings.Nietos Activos y Saludables intervention has the promise to increase Latino grandparents' knowledge about and self-efficacy in supporting grandchildren's PA. Future studies should test the impact of the intervention on grandparents' behavior and grandchildren's health outcomes using a randomized controlled trial. (PsycINFO Database Record (c) 2019 APA, all rights reserved).