The 25-year-old model confirmed in an Instagram post Tuesday that she said "yes" to Kushner after six years of dating.
"I love you more than I have words to express," Kloss captioned a photo of herself kissing Kushner on the cheek.
"Josh, you're my best friend and my soulmate. I can't wait for forever together. Yes a million times over," she wrote, adding a diamond ring emoji.
People said Kushner proposed earlier this month in New York.
"He proposed a few weeks ago during a romantic weekend together in upstate New York," a source told the magazine.
"They're both overjoyed and happily celebrating. Their hearts are full and they're excited to build their future together," the insider added.
Kloss called Kushner, the brother of Trump adviser Jared Kushner, her "ride or die" in a rare post with the businessman in April. She and Kushner largely keep their relationship out of the public eye.
"It's not like I've ever wanted to be so secretive about my private life," Kloss said in the summer issue of Porter magazine.
"I know in my life what really matters to me. I'm not trying to hide that from the world;. I just really like having a more private private life," she explained. "I've got nothing to hide, though!"