Hawaii (8-1) advances to play No. 3 Oklahoma in the second of two semifinal games Wednesday.
Valdes hit a 3-pointer with 1:47 left in the first half to give the Rainbow Warriors a 32-25 lead and Isaac Fleming added two free throws to make 34-25 at intermission.
Sai Tummala hit the second of two free throws to spark a 10-0 run that pushed Hawaii's lead to 21 points, 68-47 after Stefan Jovanovic hit a layup off a feed from Fleming with 4:27 left.
Tummala added 12 points for Hawaii and Fleming added 11 points, eight rebounds, four assists and six steals. Jovanovic added 11 points, nine boards and four blocks.
Jeremy Morgan had 13 points to lead Northern Iowa (7-4).