The attacker hacked at the right arm of a woman inside 234 East 52nd Street following an argument, police sources said.
It happened around noon, and the victim was taken to Weill-Cornell Medical Center in serious but stable condition, according to Fire Department officials.
She appeared calm as she was loaded into an ambulance with her right arm wrapped.
The slasher, who appeared to be in his late 20s or early 30s, wore a placid expression as he was arrested nearby on East 52nd Street and Third Avenue.
A roughly 9-inch blade fell out his clothing as cops collared him.
Police have not released his name.