The shooting happened just before 11 p.m..
The gunfire broke out toward the end of an annual community celebration, and sent people running for their lives.
The crowd at the celebration, which features musical performances from former residents and current local talent, was dispersing when the shooting began in a playground area, officials said.
A 38-year-old man was shot in the head and pronounced dead a hospital. He has been identified as Jason Pagan, according to a police source.
One other person is in critical condition.
Seven of the shooting victims were male and five were female, with their ages ranging from 21-55.
NYPD Commissioner James O'Neill said six of the 11 people wounded have been released from the hospital.
O'Neill says there were more than 100 officers and anywhere from 2,000 to 3,000 people at the gathering, which he called a "great event" that had been held for years.
Police recovered a gun at the playground. Investigators said they believe at least two gunmen were involved.
So far police do not have a suspect and there have been no arrests.
It is not yet clear what led to the gunfire, police say.
Mayor Bill de Blasio said that the incident "shattered a peaceful neighborhood event."
"I want to make really clear this tragedy does not define Brownsville, does not define the people of Brownsville, does not define what's happening on the ground in a community that has worked so hard and come so far," said the mayor.
State Sen. Roxanne J. Persaud expressed frustration with the shooting, which she called "unacceptable" and "cowardly."
"Our community mourns again. We should be able to have fun in open spaces without fear of violence," tweeted Persaud, whose district includes Brownsville. "Respect your community. We are better than the violence."
(The Associated Press contributed to this report.)