President Trump on Saturday named Army Chief of Staff Gen. Mark Milley to succeed Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford.

"I am pleased to announce my nomination of four-star General Mark Milley, Chief of Staff of the United States Army – as the Chairman of the Joint Chiefs of Staff, replacing General Joe Dunford, who will be retiring," Trump tweeted. "I am thankful to both of these incredible men for their service to our Country! Date of transition to be determined."

Speaking to reporters on Friday, Trump hinted an announcement related to the Joint Chiefs of Staff was forthcoming as he unveiled other picks -- William Barr for attorney general and Heather Nauert for U.N. ambassador.

"I have another one for tomorrow that I’m going to be announcing at the Army-Navy game,” Trump said. “I can give you a little hint: It will have to do with the Joint Chiefs of Staff and succession."

Trump had been expected to make the announcement at the Army-Navy game, but made the announcement on Twitter instead. Typically, the successor to the chairman would not be selected until next year, given that Dunford is due to serve until Oct. 1, 2019.

WHO IS GEN. MARK MILLEY, TRUMP'S PICK FOR JOIN CHIEFS CHAIRMAN?

Sources told Fox News on Friday that Dunford has no plans to end his term as chairman early. The next chairman will serve for four years, instead of two years. But a change was made in 2016 because Congress wanted continuity before and after a presidential election.

Dunford was nominated to the post in 2015 by former President Barack Obama. Trump nominated him for a second two-year term last year.

Milley assumed duty as the chief of staff of the Army in 2015 after serving as the commander of Army Forces at Fort Bragg in North Carolina. He has served in command and leadership positions, including in Afghanistan and Iraq.

Fox News' John Roberts and Alex Pappas contributed to this report.