Background: Latino grandparents are often involved in rearing of grandchildren. However, their roles in promoting Latino children's leisure time physical activity (LTPA) have been largely overlooked by researchers. Aim: The purpose of this study is to examine Latino grandparents' involvement in and support for grandchildren's (age 2-12) LTPA as well as issues facing Latino grandparents. Method: Participants included 53 Latino grandparents who lived with/cared for or used to live with/care for grandchildren of 2 to 12 years old in three low-income areas of Los Angeles County. Data were collected using 4 semistructured focus groups and 26 semistructured interviews. Transcribed data were systematically and iteratively coded and analyzed. Results: Latino grandparents perceived various benefits of physical activity to their grandchildren's physical and mental health, and supported grandchildren's LTPA directly and indirectly. The direct support included doing physical activity with grandchildren, taking grandchildren to places for physical activities, verbally asking grandchildren to do physical activities, and rewarding grandchildren for doing physical activities. The indirect support was made through providing suggestions to parents. Mothers usually regulated Latino grandparents' involvement in or support for grandchildren's LTPA and tended to promote a maternal lineage. Some Latino grandparents reported barriers to supporting grandchildren's LTPA, including lack of information, access to affordable and safe LTPA facilities/programs, transportation, money, and time. Conclusions: Latino grandparents are advocates for grandchildren's LTPA and support grandchildren's LTPA directly and indirectly. It may be promising to develop interventions to increase Latino grandparents' competency in supporting grandchildren's LTPA.