Results are still coming in but Mr Trump looks set to extend his lead over rivals Ted Cruz and John Kasich.
Meanwhile Democratic hopeful Hillary Clinton, a former senator for New York, is expected to defeat Brooklyn-born Vermont Senator Bernie Sanders.
Wins will put Mrs Clinton and Mr Trump closer to securing their nominations.
US networks projected the New York billionaire had won in his home state just seconds after the polls closed at 21:00 EDT (01:00 GMT), but the result for the Democrats was not immediately called.
The margin of Mr Trump’s victory and the number of delegates he secures will determine the real impact of his latest success.
Speaking at a building bearing his name in Manhattan, Mr Trump said his win had been “bigger than anyone projected even in their wildest imaginations”.
“This has been an amazing week… We’re really, really rocking.”
The voting in New York on Tuesday was marred by irregularities, including more than 125,000 people missing from New York City voter rolls.