NEW YORK (Reuters) - Four months after losing the presidential election to Barack Obama, Mitt Romney is joining his son's investment firm, a Romney adviser said on Wednesday.

Romney, last year's Republican presidential nominee, will serve as chairman of the executive committee at Solamere Capital, said the adviser, who asked not to be identified. A formal announcement is expected later this week. The move was first reported by NBC News.

Romney's son, Tagg Romney, co-founded Solamere in 2008 with Eric Scheuermann and Spencer Zwick, Mitt Romney's finance chairman in his 2008 and 2012 presidential campaigns.

Mitt Romney co-founded the private equity firm Bain Capital before serving as Massachusetts governor.

(Reporting By Edith Honan in New York and Timothy McLaughlin in Boston; Editing by Dan Whitcomb and Peter Cooney)