Water Valley (8-2, 4-0) sits alone atop Region 2-3A with one regular season game to play. Aberdeen falls to 5-5, 3-1 after losing its first division game in four years.
Aberdeen punted one time and was stopped by Water Valley on down three times in the first half.
"We should have scored 28 in the first half, just didn't make the plays," said Aberdeen coach Chris Duncan. "We played a good football team and came up a little short."
With Water Valley trailing 8-7 in the fourth quarter, Jeoffry Gordon broke through a tough Aberdeen defense for a 33-yard touchdown. Alex Robles' kick made it 14-8 with 5:03 to play.
Water Valley then recovered a fumbled kickoff, and quarterback E.J. Bounds ran it in from a yard away for the Blue Devils' second score in 15 seconds.
Aberdeen scored on Jeremy Brandon's 15-yard catch from Josh Williams with 2:16 to go, but K.J. Lee recovered the onside kick and Water Valley ran out the clock.
Justin Lucas capped Aberdeen's opening drive with a 3-yard touchdown, and Braxton Anderson caught the two-point pass.
Devin Fleming's second-quarter 20-yard touchdown pass from Bounds cut Aberdeen's lead to 8-7 at the half.
"If there's one thing we're going to do, we're going to scrap," said Water Valley coach Brad Embry. "We've learned to execute, got better, stronger and faster."