"I will vote for Hillary, I will talk to my Republican friends about helping her, and I will donate to her campaign and try to raise money for her," Meg Whitman told The New York Times.
The billionaire Whitman is CEO of Hewlett Packard Enterprise and used to be chief executive of eBay. Whitman funded her own, ultimately failed campaign to be elected governor of California in 2010.
Whitman said she stood by remarks earlier this year in which she compared Trump to Hitler and Mussolini, and said dictators often come to power by democratic means.
"Time and again history has shown that when demagogues have gotten power or come close to getting power, it usually does not end well," Whitman said.
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She also disclosed that Clinton had reached out to her about a month ago in a telephone call.
Whitman will donate an amount in the "mid-six figures" to the Clinton campaign, the Times said, quoting an aide to her.
Whitman was a top fundraiser for Mitt Romney's 2008 presidential campaign and chaired New Jersey governor Chris Christie's finance team this year until he pulled out of the race.
When Christie endorsed Trump, Whitman accused the governor of an "astonishing display of political opportunism, the Times said.