The 32-year-old reality superstar and the 25-year-old NBA pro haven't been shy about showing plenty of PDA out and about together, and are now "full-on dating," a source close to Khloe tells ET.
"They're taking it slow, day by day, but are having so much fun together," the source says about the attractive pair, adding that they met at a party and had already started speaking for a few months before they connected on their trip to Mexico over Labor Day weekend. "It’s still the honeymoon stages and they both have a huge crush on each other."
And as Khloe's mother, Kris Jenner, reinforced to ET last week, the Kardashian clan definitely approves. Tristan has apparently won over the famous family by being smitten with Khloe, and not being into the "celeb scene."
"He’s the nicest, sweetest guy," the source says. "He comes from a very strong family and has a tight circle of close friends. He really isn't into the 'celeb scene' and does his own thing. He's close friends with [his Cleveland Cavaliers teammate] LeBron [James] and a couple of his teammates."
Still, Khloe -- who's previously dated NBA stars Lamar Odom and James Harden -- is of course proceeding with caution.
"She's very happy but taking it slow and not making any rushed judgments," the source says.
As for Khloe's complicated relationship with Lamar, the source says it's now pretty much non-existent. Khloe filed for divorce from Lamar for the second time in May.
"Lamar is out of her life completely," the source says. "No animosity there -- she still cares about him, she still wants him to be safe, but it's over."
"Khloe is very concerned but doesn't know what else she can do at this point," the source said, referring to reports that the former Los Angeles Lakers star was still struggling with alcohol issues. "She's tried everything she can to help him. The man was on his deathbed and she helped him get a second chance at life. If that didn't wake him up, what will?"