The world's most prestigious marathon, which is now due to take place on Sept. 14, generally draws more than 30,000 runners from all over the world ranging from decorated professionals and Olympians to amateur runners.
"Our expectation - and it's an expectation and a hope right now - is that this date will get us to a safer place in relation to the spread of the coronavirus," Marty Walsh said during a news conference.
"Our priority right now is making sure the health and safety of the runners, of the fans, of the medical personnel, first responders, residents of the Commonwealth of Massachusetts (and) visitors from around the world."
The Rome, Paris, Barcelona and Rotterdam marathons have all been postponed or canceled, as have the World Half Marathon Championships in Gdynia, Poland.
The Tokyo Marathon went ahead on March 1 with elite runners only. The London Marathon is currently still on for April 26.
The decision to postpone the Boston Marathon comes three days after Governor Charlie Baker declared a state of emergency in Massachusetts.