Sources told the New York Times that difficulties raising money, bought about by his decline in the polls, mean Walker is no longer vying for the Republican presidential nomination.
He was towards the top of the polls after officially announcing his campaign in early July. But he's since lost ground, amid the rising popularity of business mogul Donald Trump, as well as a series of gaffes.
Walker gave shifting answers to questions about illegal immigration, and once suggested a wall between the United States and close ally Canada might be in order, in an apparent effort to double down on rivals' calls for a wall on the Mexican border.
He is the second Republican candidate to drop out of the race, after former Texas governor Rick Perry stepped down earlier this month, having languished near the bottom of opinion polls.