Hillary Clinton thanked her supporters for helping her reach a historic moment for women – the US Democratic nomination for president.
“Thanks to you, we’ve reached a milestone,” she told cheering crowds at a rally in New York.
She hailed “the first time in our nation’s history that a woman will be a major party’s nominee”.
Earlier, Mrs Clinton won the Democratic primary in New Jersey, cementing her hold on her party’s nomination.
She went on to win South Dakota and New Mexico, while her rival Bernie Sanders found victory in the Montana and North Dakota caucuses.
Six states have been voting in primaries on Tuesday but the race in California will count the most.