The shooting took place on Sunday just before 6 p.m. on the southbound 1, 2, 3 platform at 14th Street and Seventh Avenue station.
The 24-year-old victim walked to Lenox Hill Hospital. He was later transported to Bellevue Hospital in grave condition, according to police.
No further information was released on a possible suspect. Police were reviewing surveillance footage of the area of the shooting.
There have been several violent attacks on the subway system recently.
Earlier this month, a subway rider was hospitalized after being stabbed in the neck when he refused to give a homeless man a dollar in the Herald Square subway station.
Also this month, a man was found stabbed to death inside the Chambers Street Station in Lower Manhattan.
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