Harold Rivera had last been seen around 1 p.m. outside of Black Diamond Middle School at 4730 Sterling Hill Drive.
Police said Harold is a student at Deer Valley High School but has the cognitive ability of a fourth-grader and was considered at-risk because of his age and mental state.
Shortly before 7 a.m. today, Antioch police wrote on Twitter that the teen has been found and reunited with his family. More details about when and where he was found were not immediately released.