Kyle Spain added 13 points for the Aztecs (23-8), who all but assured themselves of a trip to the NCAA tournament with a stirring second-half effort against the Cougars (25-7), who haven't won the conference tournament since 2001 despite being the top seed in each of the last three years.
Wade was outstanding for the second straight night in his final conference tournament, scoring 12 points in the final 6:13. The Aztecs have won five straight since losing at home to BYU on Feb. 24.
San Diego State will go for its third Mountain West tournament title since 2002 on Saturday against the winner of second-seeded Utah's late semifinal against Wyoming at the Thomas & Mack Center.
Jimmer Fredette scored 17 points on 5-of-14 shooting and Lee Cummard added 11 points, but BYU's five-game winning streak ended when the Cougars left too many points on the free throw line after halftime.
Although BYU controlled play for long stretches of the second half, the Aztecs hung around thanks to BYU's six missed free throws, including three by center Chris Miles in the final 4:45. The Cougars also went 5-for-18 on 3-pointers, disappointing the overwhelmingly pro-BYU crowd in Las Vegas.
BYU, which had its five-game winning streak snapped, was the only team to beat San Diego State twice during the regular season. The Aztecs finished the conference slate one game behind co-champions BYU, Utah and New Mexico despite their late surge.
Wade and Spain led the Aztecs in scoring during the season, but Wade has emerged as their clear go-to guy in tourney time. Wade had 20 points and 10 rebounds in the quarterfinals as San Diego State posted a surprisingly easy win over host UNLV.
San Diego State nursed a small lead through most of the game until BYU went ahead with 7:28 to play on a 3-pointer by Fredette. Wade immediately put the Aztecs back in front with five straight points, making a graceful dunk on an offensive rebound of Spain's shot and following it with a three-point play.
Jackson Emery put the Cougars ahead 57-56 on a 3-pointer with 2:31 left, but Wade promptly answered with his own 3-pointer from straightaway. Wade then came off a screen for a tough layup with 26.3 seconds left to put San Diego State up 61-58, but Richie Williams was called for a foul on Fredette's long jumper with 10.6 seconds left.
After a lengthy video review determined Fredette had his foot on the 3-point line, he hit two free throws. Spain hit one of two on the other end, but the rebound went out of bounds off a BYU player, and Wade made two more free throws.
After Cummard's length-of-the-court layup with 1.6 seconds left, San Diego State's inbounds pass deflected free into the corner, but BYU's desperation shot wasn't close.
Williams, a 5-foot-10 guard, led San Diego State with eight rebounds.