Demercado visited the Horned Frogs' campus this weekend and announced his commitment via Twitter.
"I felt at home," Demercado told Jeremy Clark of 247Sports.com.
"I could tell that they are just a big family. From the coaches, to the players, to the staff. Everyone welcomed me and my mom with open arms and family is huge to me so that played a great factor in my choice."
Demercado rushed for 1,026 yards and 15 touchdowns in 11 games for Saddleback College, which is in Mission Viejo, Calif., in 2017. He also caught 54 passes for 443 yards and two touchdowns.
Demercado told 247Sports that he plans to be on campus in June. He'd be available for the 2018 season.
"I loved it ! Everything was great, the stadium, the people, the players, the staff," he said. "I was just blown away by everything."