The Saturday planting blitz took place at parks throughout Staten Island, Brooklyn, Queens and the Bronx.
About 3,500 trees were planted in Van Cortlandt Park alone.
Mayor Michael Bloomberg has set a goal of planting one million new trees in the city by 2017. Nearly 400,000 have been planted so far.
In addition to providing shade, trees emit clean oxygen and cool the city during the summer.
The effort comes on the heels of a September storm that spawned two tornados and knocked down thousands of city trees.
The city is encouraging New Yorkers to donate to the MillionTreesNYC campaign to fund replacements.