Harris, 26, confirmed rumors of a the couple's reconciliation Friday on Twitter, People magazine reported.
"Yes I am his #1 girl again," she said. "Yes we are happy. Hope that clears up any confusion! Xo."
The couple broke up in June 2011 just days before they were scheduled to exchange vows in front of 300 guests at the Playboy Mansion in Los Angeles.
Harris at the time criticized Hefner's skills in the bedroom on Howard Stern's radio talk show, and Hefner, 86, expressed relief, saying "I missed a bullet."
Harris later issued an apology for her comments about Hefner's sexual skills.