Speaking to The Atlantic's "The Ticket" podcast, the Hewlett-Packard CEO said that presidential elections are "binary" and that she won't vote for a third party.
"I've been very clear that I can't support Donald Trump," she said, which means that Biden will be getting her vote.
She called Biden a "person of humility and empathy and character," something that she said the country needs right now. She further added that character is important.
"As citizens, our vote is more than a check on a box. You know, it's a statement about where we want to go, and I think what we need now actually is real leadership that can unify the country," she said.
The Hill writer Tal Axelrod posited that Fiorina's endorsement of Biden is likely to be "dismissed." However, Axelrod also noted that Fiorina is hardly a left-leaning Republican. She ran as a Tea Party candidate for the California Senate in 2010, signaling her allegiance to a group of right-leaning Republicans that, by some measures, lean even further right than the mainstream Republican Party of its time.
Further, she was briefly Ted Cruz's prospective running mate in 2016, before the GOP nomination went to the Trump/Pence ticket.
What's more, Fiorina stood by Trump, casting her ballot for him in 2016 even after he publicly mocked her, asking voters how anyone could vote for someone with a face like hers.
In her recent interview, Fiorina also chided her fellow Republicans for sticking with Trump despite his shortcomings. However, she noted that many Republicans believe that hitching their wagon to Trump will keep them in their jobs.
"I've been public in my disappointment about how few politicians have been willing to stand up and speak on values and principles. But I understand it if you're in politics, sadly," she said.
Fiorina joins a growing list of Republicans who are backing Biden in 2020.
As Reuters reported earlier this week, "dozens" of Republican former national security officials have jumped ship from Trump and backed Biden.
According to an unidentified source, John Bellinger III and Ken Wainstein, who both held positions within the George W. Bush administration, lead the group, which holds that Biden is the better candidate for the country even though Trump aligns more with them policy-wise.
"Trump pals around with dictators. He's a real danger," a person involved in the group said, speaking on condition of anonymity.