The 27-year-old New York state resident was securing the line on the 145-foot tugboat when it cinched his arm just below the elbow and severed it about 11:11 p.m., police said.
Officer Paul Hessian with the NYPD's Harbor Unit Charlie got to him and tied a tourniquet just below his left shoulder to stop the bleeding so police could bring him back to the Brooklyn Army Terminal and then Lutheran Medical Center, police said.
The deckhand, whose name wasn't immediately released, was listed in stable condition, police said.
It wasn't immediately clear if the arm was recovered.
Mister T, which sources said is registered to a Connecticut-based company, was built in Rhode Island in 2001, according to the company's website.