This year, organizers widened the number of entries from 27,000 last year to 36,000, close to the record 38,708 who ran in 1996 on the centenary of the race.
There will be more than 35,660 runners, 5,330 of whom come from 70 countries outside the United States.
Last year's win by Ethiopia's Lelisa Desisa passed almost unnoticed.
He returns to Boston, one of the six biggest foot races in the world and part of the prestigious World Marathon Majors circuit.
The 24-year-old has met several victims of the attacks calling them "an inspiration" and has said he will be running again to show that he has no fear.
Several former winners are also taking part on Monday, including America's Joan Benoit Samuelson, who won in 1979 and 1983, and Amby Burfoot (1968), who last year failed to complete the race in the wake of the attacks.
Josef Klobusnik, a Slovakian living in Minnesota, said he was still hurting because of what happened.
"Last year was very painful for me, I had a lot of friends here," he told AFP.