"I’m here to tell you now that I am going to get in," Sanford told Chris Wallace on "Fox News Sunday.”
In a tweet, Sanford outlined his reasons for running.
"We have a storm coming that we are neither talking about nor preparing for given that we, as a country, are more financially vulnerable than we have ever been since our Nation’s start and the Civil War. We are on a collision course with financial reality," he posted.
The former governor said he plans to make a formal announcement in South Carolina next week.
Sanford said he's launching a long shot bid against Trump, who has wide support among Republicans, because he believes "we need to have a conversation on what it means to be a Republican. I think as a Republican party we have lost our way."
On Sanford's web site, he said the country is heading in the wrong direction. He also tweeted, "Essentially no one 'leading' in Washington is leading, or even speaking of, our financial predicament."
"We’re headed towards the most predictable financial crisis in the history of our country,” he said.
In response to Sanford's announcement, Trump campaign communication's director Tim Murtaugh gave ABC News a one-word reaction: "Irrelevant."
Sanford's tenure as South Carolina governor was rocked by scandal in 2009 after he secretly traveled to Argentina to meet with his lover, Buenos Aires resident Maria Belen Chapur. He confessed to having an extramarital affair in a news conference after his return.
He finished his term as governor and was elected to Congress several years later.
ABC News' Theresa Cook, Eric Noe and Huma Khan contributed to this report.