The Terrapins held Denver to two points over the final 6:06 and overcame what had been a nine-point deficit in the second half.
Maryland moved into the NIT quarterfinals, where the Terrapins will meet the winner of Saturdays' game between Alabama and Stanford. Maryland is one win away of making a trip to New York for the NIT's Final Four.
Walls scored 19 points, making 7-of-10 from the field and all five of his free throw attempts. Seth Allen added 13 points for the Terrapins (24-12) and Nick Faust had 12.
Denver (22-10), which finished second in the regular-season Western Athletic Conference race only to fall in the quarterfinals of the league tournament, was led by Chris Udofia with 24 points.