Running back DeJuan Green had a 25-yard touchdown run in the second overtime to give South Florida (5-3, 3-2 Conference USA) the lead.
Selmon, the son of former NFL great and current South Florida athletic director Lee Roy Selmon, intercepted a pass by Gino Guidugli on the next possession to end the game.
Cincinnati (4-4, 2-3) scored on the first play of the first overtime, a 25-yard pass from Guidugli to Dennis Hart.
The Bulls tied it on a 5-yard run by quarterback Brian Fisher.
South Florida's Santiago Gramatica tied the game 10-10 with a 30-yard field goal with 3:33 remaining.
The victory sustained postseason hopes for South Florida, which has two victories against Division I-AA teams.
The Bearcats deprived South Florida of a conditional bid to the Hawaii Bowl last year by winning their season finale at East Carolina.
Cincinnati opened the scoring on a 2-yard pass from Guidugli to Mike Daniels with eight seconds left in the first quarter. The play capped a 76-yard drive that took 15 plays and more than seven minutes.
Cincinnati threatened to double the lead on its next possession, but backup quarterback George Murray was stopped in the backfield on a fourth-and-1.
South Florida, which produced only 1 yard of offense in the first quarter, had three quarterbacks combine for 11 yards passing in the first half and trailed 7-0.
Chet Ervin's 38-yard field goal gave Cincinnati a 10-7 lead entering the fourth quarter.
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Arizona linebacker Scooby Wright's left knee injury is not season-ending, according to an MRI Wright had after Thursday night's opening win over UTSA, sources told ESPN's Brett McMurphy.
Syracuse starting quarterback Terrel Hunt left the Orange's season opener Friday night against Rhode Island with a lower right leg injury and won't return.
The Mountain West told ESPN's Brett McMurphy that officials correctly ruled the final play that kept Colorado from getting a potential game-tying touchdown against Hawaii before time expired.
ESPN 300 running back Elijah Holyfield, the son of former heavyweight boxing champion Evander Holyfield, has decided to stay home and attend Georgia, he announced Friday.
Miami reserve linebacker Al-Quadin Muhammad has been suspended for the season opener against Bethune-Cookman on Saturday for a violation of team rules, a source confirmed to ESPN.com
All-American defensive end Shawn Oakman and safety Orion Stewart have been suspended for Baylor's season opener Friday against SMU for a violation of team rules, a school spokesman told ESPN.com.