Jordan Williams made 1 of 2 from the line to put North Texas up one with 1:50 left in overtime, but Southern Miss (6-12, 1-6 Conference USA) scored the next six points - including Armelin's trey - for the win.
Armelin was 10 of 17 from the floor, including 4 of 9 beyond the arc. Norville Carey added 18 points and 12 boards.
North Texas (8-11, 2-5) took advantage of two 3-pointers by Maurice Aniefiok late in the first to lead 37-29 at the break.
Southern Miss came back and finally tied it at 55 with 4:02 left on Matt Bingaya's three-point play. Carey's 3-pointer with four seconds left forced overtime.