"They have amicably split but remain good friends who still greatly care for one another," a rep for Minnillo told the magazine.
A rep for Lachey, a former member of 98 Degrees, also confirmed the news.
Lachey and Minnillo began dating in 2006, but rumors of a breakup had dogged the couple recently. Earlier in June, Minnillo told People that her relationship was "very much still on."
MTV News most recently caught up with Lachey, 35, and Minnillo, 28, in May on the red carpet at the Maxim Hot 100 party in Los Angeles, where the pair joked together and compared memories of their high school proms.
Lachey and Minnillo started a relationship after Lachey's high-profile 2005 split from wife Jessica Simpson. Following the breakup of his marriage, which had been chronicled on the MTV reality show "Newlyweds," Lachey released the first single off his solo album, "What's Left of Me." Minnillo starred in the video.
A source told People that Lachey and Minnillo are "still friends. It was the right thing for both of them to do."