UFC president Dana White announced the matchup on SportsCenter. Jones, 28, was originally scheduled to fight defending champion Daniel Cormier on April 23 at MGM Grand Garden Arena, but Cormier recently withdrew because of a lower leg injury.
Jones (21-1) hasn't fought since he defeated Cormier via unanimous decision at UFC 182 in January 2015. The victory marked Jones' eighth consecutive title defense, a record for the 205-pound division.
He was scheduled to defend the title against Anthony Johnsonin May but was arrested one month before the fight on a felony hit-and-run charge in Albuquerque, New Mexico. The UFC stripped Jones of the title shortly after his arrest. Jones eventually entered a plea agreement on the case and was sentenced to 18 months supervised probation.
Once Cormier withdrew from the event, the UFC initially turned to Johnson (21-1) as a replacement, but Johnson was unable to accept due to a recent medical procedure.
"Obviously, the No. 1 choice would be Anthony Johnson, who is the No. 2-ranked guy in the world and a fight that everybody wants to see," White said. "But Anthony Johnson just had surgery on his mouth and can't even put a mouthpiece in for at least three weeks."
That opened the door for Saint Preux (19-7), who is 7-2 in the UFC. A former college football player at Tennessee, Saint Preux, 32, is coming off a decision win overRafael Cavalcante in February. He trains out of Knoxville, Tennessee.
Although Saint Preux doesn't carry the same history with Jones as Cormier, he is the UFC's sixth-ranked fighter in the weight class.
"The one thing we've learned in the 16 years in this business is you never know what's going to happen," White said. "Anything is possible when two guys get in there. Jon Jones and OSP have never faced off before. No. 1 versus No. 6. Two big, strong, athletic, explosive guys. Anything is possible."
Cormier (17-1) won the official title by submitting Johnson at UFC 187. He has recorded one successful defense, a split decision against Alexander Gustafsson in October.
In a statement to ESPN, Cormier said his recovery time is estimated at four to six weeks.
"The decision not to fight on April 23 was one of the hardest decisions I've had to make," Cormier said. "I've been scheduled to fight 18 times, and I have made that walk every time. I want to apologize to Jon Jones, Lorenzo Fertitta, Dana White and all of the UFC fans for not being able to defend my title.
"The recovery should not take very long, and I can't wait to compete and defend my title again. For all the fans, thanks for your support. I appreciate each and every one of you."
White explained on SportsCenter the decision to make Jones-Saint Preux an interim title fight.
"Thing is, Jon has never lost his title in the Octagon," White said. "He was stripped of his title because of problems outside. We think it's the right thing to do. It's not his fault that Cormier is injured so. He will fight for the interim light heavyweight title. When Cormier comes back, they'll decide who the real champ is."