The guard, a native of North Carolina, becomes the first member of the Terrapins' 2018 class.
The Terps will begin their third NCAA baseball tournament appearance in four seasons this weekend after being loosened up with Coach John Szefc's help.
Maryland had played area rival Georgetown in the first two years of the series.
Jackson is Maryland top returning scorer and rebounder.
The freshman forward has until Wednesday's deadline to decide on his future.
The Terrapins posted a program-best multiyear score of 984.
Lewis will have three years of eligibility remaining after sitting out this season due to NCAA transfer rules.
Maryland will meet the winner of Bryant and Monmouth next weekend.
It is the fifth consecutive year the Terrapins will enter the tournament as the top overall seed.
Junior Connor Kelly scores a career-high four goals to lead the Terrapins to a 10-9 win.