Smokers will no longer be allowed to light up in parks, beaches, public plazas and on boardwalks.
The Bloomberg administration says the ban will protect people from second-hand smoke and help keep parks and beaches clean.
Some health experts questioned whether the smoke poses a serious danger in open, outdoor spaces.
Critics of the law say the city is trampling on civil liberties.
Those who break the law could face fines of $50 per violation.
However, the city has said it plans to rely on signs and social pressure instead of active enforcement.
The expansion comes almost a decade after the city banned /*smoking*/ in restaurants, bars and other public indoor spaces.
Smokers who want help quitting can visit www.nyc.gov or call 311. NYC Quits on Facebook has also become an online smoking cessation support network with more than 5,000 fans.
