The televised event will take place at the Cross Insurance Center. The 205-pound matchup was first reported by the Bangor Daily News.
The announcement comes just four days after Bader and St. Preux each recorded wins at UFC 174 in Vancouver. At the post-fight news conference, both light heavyweights expressed an interest in fighting the other next.
"Let's do it," Bader said. "He's on a (five-) fight win streak. Outside of (UFC champion) Jon Jones, that's the longest streak in the division. It's tough to do -- there are a lot of killers. We fought on the same card and both got wins."
Bader (17-4) defeated Rafael Cavalcante via unanimous decision at UFC 174. The former collegiate wrestler improved his UFC record to 10-4.
He will take a two-fight win streak into the August bout. Prior to the Cavalcante win, Bader outpointed Anthony Perosh in a three-round decision in December.
St. Preux (16-5) is coming off a second-round submission over Ryan Jimmo. Fighting out of Knoxville, Tennessee, St. Preux is 4-0 in the UFC and has won 13 of his last 14.