"Well, I've said always that I would endorse the Republican nominee, and he's about to become that," Forbes told CNN's Poppy Harlow on "New Day."
When Harlow asked whether Forbes was publicly endorsing Trump, he replied, "I'll be supporting him."
Forbes, who ran as a Republican candidate for president in 1996 and 2000, described the presumptive Republican nominee as "absolutely unique."
He acknowledged that although both Trump and Hillary Clinton have "extremely high negatives," Forbes said her ratings are "a lot more frozen" given the amount of time she has been in public office.
Trump, he told Harlow, can "assuage the doubters" with his choice of a running mate -- a decision Trump has said he will make within the next week.
"If it's somebody that people say, 'Hey this is a person with experience, not on the original Trump team, hey, maybe this guy can put it together,'" Forbes said. "That's what you have to do now -- assuage the doubters."
When asked who's Forbes' top pick for vice president among the five currently considered his top choices -- former Speaker of the House Newt Gingrich, Gov. Chris Christie, Gov. Mike Pence, Sen. Jeff Sessions and retired General Michael Flynn -- he said he's "not sure it's going to be any of them."
"He hasn't made his mind of up yet, so I don't know who he's going to pick," Forbes said. "I'm not going to bet a penny of my 401k on any of those."
Forbes floated the names of Sen. Rob Portman, former Indiana Gov. Mitch Daniels and Gov. Nikki Haley as possible alternative picks for Trump's running mate.