The Flyers will face No. 23 Xavier in the title game. It's the second time in three years that Dayton (5-0) has started the season by winning its first five.
Monmouth fought back midway through the second. However, Chris Brady missed the free throw on a potential 3-point play that would have tied at 68 with 1:52 to play.
Smith then buried a 3-pointer 20 seconds later.
Dayton opened the second half with a 10-0 run to go up 47-32 with 14 minutes left.
Monmouth and USC will play in Sunday's third-place game.