Smith -- the second-leading rusher in the country going into the game -- had 29 carries, including an 87-yard touchdown run. He also caught four passes for 48 yards.
Marshall (1-8, 1-4 Conference USA) got a 45-yard touchdown run from Chubb Small and two field goals from Anthony Binswanger.
Quarterback Kyle Israel completed 14 of 21 passes for 203 yards and two touchdowns for UCF (6-3, 4-1). Both of his scoring passes went to redshirt freshman Brian Watters.
Smith's 87-yard run came on a trap play up the middle. The run gave UCF a 20-0 second-quarter lead and made Smith the only running back in Division I with two 80-plus yard touchdown runs this season.
UCF freshman Darrin Brown returned an interception 37 yards for a touchdown to complete the scoring. That was one of three interceptions for the Knights, all in the second half.