Bieber, 19, is the first recording artist to reach that milestone, E! Online reported.
His 200th encounter was with Annalysha Brown-Rafanan, 8, of Atlanta, who suffers from a liver disorder that could kill her. Annalysha questioned Bieber during their meeting last weekend and then asked if he would marry her, a proposal he accepted.
The foundation used Twitter to thank Bieber "for granting so many amazing wishes."
"Hey Beliebers! Join us in congratulating @JustinBieber on his 200th wish & help us grant even more," the foundation said.
Bieber, in his response, called the meeting "one of the best moments I've ever had."
Discovered on YouTube in 2008, Bieber in recent months has endured a lot of bad publicity, including accusations of reckless driving around his gated community in California and a fight outside a nightclub in a New York beach resort involving some of his entourage.