'I certainly cannot in any way support President Trump this year,' Powell, a Republican, told CNN's Jake Tapper on 'State of the Union,' adding that he couldn't bring himself to vote for Trump four years ago.
The retired general voted for Trump's Democratic opponent, Hillary Clinton, in 2016, and hacked emails released in September of that year showed Powell strongly condemning Trump, labeling him a 'national disgrace and an international pariah.'
Powell said Sunday that he is 'very close to Joe Biden on a social matter and on a political matter,' adding: 'I worked with him for 35, 40 years, and he is now the candidate and I will be voting for him.'
Biden expressed his thanks in a tweet later Sunday, saying, 'This isn't about politics. This is about the future of our country. Grateful for your support, Secretary Powell.'
Asked by Tapper if he would be campaigning for Biden, Powell said he hadn't been asked to do so and that he doesn't think he will be. 'Campaigning is not my strong suit, and I will be speaking for him, but I don't plan to make campaign trips,' he said.
The former diplomat said Sunday that the ongoing and widespread protests over the police killing of George Floyd, a black man in Minneapolis, and Trump's strongman approach to the national unrest is evidence of a growing opposition to his presidency.
'I think what we're seeing now, this massive protest movement I have ever seen in my life, I think it suggests the country is getting wise to this and we're not going to put up with it anymore,' Powell told Tapper.
Trump swiftly lashed out on Twitter following the interview, calling Powell 'a real stiff' and Biden 'another stiff.' The President also attacked Powell for his role in presenting US' case against Iraq to the United Nations in 2003.
Biden, who has been the presumptive Democratic nominee since Vermont Sen. Bernie Sanders dropped out of the race in April, has earned enough delegates to officially secure the party's nomination, CNN projected on Saturday.
His primary victory came after a win in Guam on Saturday, which allowed him to surpass the necessary 1,991 delegates to claim the nomination on the first ballot of the party's convention, set to be held in August. Biden currently has 1,992 delegates, according to CNN's tally.
This story has been updated with additional developments Sunday.