WASHINGTON—President Trump said in a tweet Thursday that he never asked then-White House counsel Don McGahn to fire special counsel Robert Mueller, directly contradicting a detailed account in Mr. Mueller’s report.

“As has been incorrectly reported by the Fake News Media, I never told then White House Counsel Don McGahn to fire Robert Mueller, even though I had the legal right to do so,” Mr. Trump wrote on Twitter. “If I wanted to fire Mueller, I didn’t need McGahn to do it, I could have done it myself.”

The special counsel’s report on Russian interference in the 2016 presidential election said that Mr. Trump “called McGahn and directed him to have the Special Counsel removed.” The report, which the Justice Department released last week, relied on interviews that Mr. McGahn gave Mr. Mueller’s team in March 2018, after which Mr. Mueller concluded that “McGahn is a credible witness with no motive to lie or exaggerate given the position he held in the White House.”

Mr. Mueller also wrote the president “made clear” to his chief of staff and chief strategist at the time—Reince Priebus and Steve Bannon, respectively—that he was considering firing the special counsel. Both men spoke to the special counsel’s investigators.

Mr. Trump declined to answer questions about obstruction of justice from Mr. Mueller and declined to sit for an interview with him.