It happened on a northbound train at the 168th Street station shortly before 5:00 p.m.
The MTA says an unknown substance was sprayed on one of the subway cars at the station.
Six people, including three children, were being evaluated at NY Presbyterian Hospital with minor injuries.
The 168th Street train station was evacuated as a precaution.
Service resumed with residual delays around 5:30 p.m.