The additional runners will add to the logistical challenges for race organizers, who are expecting more security measures in light of this year's bombings, The Boston Globe reported Thursday.
The larger number of spots will allow qualified runners more opportunity to register for a number in the 2014 marathon.
Runners from around the world have expressed interest in the 2014 race as a show of support for this year's competition, which was marred by death and injuries when two bombs exploded near the finish line.
"Rolling admissions" will begin Sept. 9, with registrations accepted until all openings are full.
Registration will be conducted entirely online at baa.org, the Boston Athletic Association, the race organizer, said in a statement.
Runners who qualify for their age and gender by 20 minutes or more will register first. Those who qualify by 10 minutes or more can register Sept. 11. Registration will reopen Sept. 16 if space remains, and again on Sept. 23 for anyone who meets the qualifying standards.
Registration will remain open until the field size is reached.
The race will be held April 21.