Expos president Tony Tavares told reporters of the move after the Expos' final home game against the Flordia Marlins, ending a two-year search for a new home.
That news was later confirmed to Washington mayor Anthony Williams by MLB officials.
It is the first North American baseball franchise relocation since the American League's Washington Senators became the Texas Rangers 33 years ago.
The Expos ran into financial difficulties since a labour dispute disrupted the 1994 World Series when they were National League title contenders. The club has been financed by the other 29 MLB club owners for the past two years.
The Expos will be renamed and plan to play the next three seasons at RFK Stadium, while a new $400 million ballpark is being built at a location along the Anacostia River waterfront near M and South Capitol Streets - now the preferred site among the four proposed in the district last year.
The deal is dependent on the district's city council passing the funding for the new ballpark, including a $13 million package to refurbish 43-year-old RFK Stadium.
It must also be approved by a 75 per cent vote of the owners, which is considered to be a formality.
