The Jackson Square T stop was closed for more than an hour and a half on Thursday as police investigated a reported stabbing.
The victim, identified as a 17-year-old man, was being treated for life-threatening injuries, police said.
The incident was first reported at the Orange Line stop in Jamaica Plain at 3:20 p.m. The station closed about 20 minutes later and reopened at 5:18 p.m., according to police and the MBTA.
Commuters on social media earlier expressed frustration with the T over delayed service.