Three people were killed and more than 100 injured in Monday's blasts. As of Tuesday morning, no one had claimed responsibility for the attacks.
"There was an explosion by the finish line about 5 minutes after I finished," McIntyre, a Needham, Mass., native, posted on Twitter Monday afternoon. "I'm ok but I'm sure there are many hurt."
Before McIntyre tweeted his condition, band mate Donnie Wahlberg took to Twitter to plead for an update on his whereabouts.
"Please reach out if you hear that @joeymcintyre is ok!!!!! Please say a prayer for everyone in Boston!" tweeted Wahlberg, who grew up in the Dorchester section of Boston.
He later wrote: "Just heard that @joeymcintyre is ok! Thanks to everyone for tweeting! Scared for everyone else in Boston. #PrayerForBoston please."