Police are asking for the public’s help tracking down the person who allegedly stole a woman's cellphone on the subway.

They say the man snatched the phone out of the 25-year-old woman's hands while standing on a packed Q train in Union Square.

Investigators say a second person, identified as a 15-year-old boy from the Bronx, was arrested. He is accused of blocking the woman from going after the suspected thief.

The teen has been charged with grand larceny.