Whitman said in a LinkedIn post on Tuesday night that, "Donald Trump's demagoguery has undermined the fabric of our national character."
"As a proud Republican, casting my vote for president has usually been a simple matter," Whitman wrote. "This year is different. To vote Republican out of party loyalty alone would be to endorse a candidacy that I believe has exploited anger, grievance, xenophobia and racial division."
Whitman, a billionaire who waged an unsuccessful gubernatorial campaign in California in 2010, said that the GOP presidential nominee's "reckless and uninformed positions on critical issues - from immigration to our economy to foreign policy - have made it abundantly clear that he lack both the policy depth and sound judgment required as president."
"Therefore, I have decided to support Hillary Rodham Clinton," she wrote, adding that it was clear that the Democratic nominee's temperament, experience and commitment to U.S. "bedrock national values" made her a better candidate.