The couple's spring fling (they began dating in April) has taken an autumnal fall, a source confirms to PEOPLE.
"They're still talking though," the insider adds. "It wasn't the perfect fit for her."
E! News first broke news of the split Wednesday.
"For all intents and purposes, Khloé is single again," a source told E!, saying Montana, 29, "won't really accept it."
"Khloé thinks he's a great guy, but it just wasn't working out," the source added.
The two have spent a lot of quality time together over the summer: Montana was on hand to help Kardashian celebrate turning 30, even posing for family photos alongside Kim Kardashian and Kris Jenner.
In August, Kardashian, 30, appeared in Montana's "Don't Panic" music video, saying the shoot was "really fun" and that working with her boyfriend was "easy and comfortable." And the rapper is featured prominently in the trailer for Kourtney & Khloé Take the Hamptons.
But Kardashian may have hinted at a breakup with her cryptic Instagram post on Thursday that quoted poet Robert M. Drake.
"Maybe one day we'll find that place where you and I could be together. And we'll catch our dreams within the waves of change," she wrote. "So smile for me one last time and believe that we'll meet again. Until then, I'll be missing you."
Reporting by JEN GARCIA