"Ultimately, it will come out of what's happening with Mexico ... and we will be in a form reimbursed by Mexico, which I've always said," Trump said.
During the interview, which took place at the White House this morning, Trump said that Mexico would pay the U.S. back "100 percent."
He confirmed that U.S. taxpayer dollars would be used to start the construction but said reimbursement would follow.
"All it is, is we'll be reimbursed at a later date from whatever transaction we make from Mexico," he said. "I'm just telling you there will be a payment. It will be in a form, perhaps a complicated form. What I'm doing is good for the United States. It's also going to be good for Mexico. We want to have a very stable, very solid Mexico."
When asked about the start of construction, Trump said it would happen in "months."
"As soon as we can, as soon as we can physically do it," he said. "I would say in months, yeah. I would say in months -- certainly planning is starting immediately."
In the hours after the ABC News interview took place, Trump issued two executive orders, one of which is about the commission of the border wall. The order calls for the secretary of homeland security to "take all appropriate steps to immediately plan, design, and construct a physical wall along the southern border."
The order calls for agency and department heads to "identify and quantify all sources of direct and indirect Federal aid or assistance to the Government of Mexico." It does not specifically state how or if those payment sources will be connected to the payment of the wall.
Trump talked about these orders during a speech he gave at the Department of Homeland Security this afternoon, saying that "Beginning today, the United States of America gets back control of its borders."
Talking about his executive orders, Trump predicted that they will "save thousands of lives, millions of jobs, and billions and billions of dollars."
Catch more of the exclusive one-hour prime-time special "President Trump: The First Interview" with David Muir tonight at 10 p.m. ET on ABC.