Officials said 35-year-old woman's right arm was severed at the bicep, above the elbow, just before 12:20 p.m.
Thursday near Fifth Avenue and 55th Street.
Two fingers were also severed from her left hand.
The woman was taken to Bellevue Hospital for surgery. She is expected to survive but will likely lose her arm.
Detectives are searching for her 35-year-old husband, who fled on foot.
The knife was recovered at the scene, and the investigation is ongoing.
Man allegedly chops off wife's arm in Brooklyn