A man and a woman, who are either dating or married, got into an argument at around 6:20 p.m. on a southbound No. 5 train as it pulled into the 59th Street station.
The woman pulled out a can of pepper spray and sprayed it at the man while still on the train.
Dozens of people fled the train and the platform, many complaining about itchy and water eyes.
Nobody was seriously injured.
It's not clear if the woman will be charged.