According to McCain's campaign website, Romney will join the senator in Mesa, Arizona for a town hall meeting on Friday, June 4th.
McCain, a four-term senator, is leading former Scottsdale Congressman J.D. Hayworth in a close race in the Republican primary in Arizona. The primary will be held in August.
The announcement may come as a surprise, as during the 2008 presidential race Romney and McCain, both seeking the Republican nomination, appeared to be bitter rivals.
The day before the Florida GOP primary, Romney slammed McCain by saying, "If you ask people, 'Look at the three things Senator McCain has done as a senator,' if you want that kind of a liberal Democratic course as president, then you can vote for him."
McCain quickly responded to the accusations with this: "The truth is, Mitt Romney was a liberal governor of Massachusetts who raised taxes, and managed his state's economy incompetently, leaving Massachusetts with less job growth than 46 other states."
Yet with Romney endorsing McCain both in the 2008 general election and earlier this year in this race, the past looks to be behind them.
McCain recently stepped up his campaign by launching a series of radio and television ads blasting Hayworth for his views on congressional earmarks.