It happened just before 4 p.m. at the Smith Street/9th Street station on the F/G lines.
Police say two men became involved in some sort of dispute, and when the 48-year-old bystander intervened, he was stabbed in the left arm.
The suspect fled the scene.
He is described as a black male in his 20s, last seen wearing a green shirt, green pants, lime green sneakers and aviator sunglasses.
The victim as taken to Methodist Hospital for treatment.
There were brief service changes and delays in F and G train service because of the investigation.
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