"Steve pretends to be at war with the media, which he calls the opposition party, yet he spent his time at the White House leaking false information to the media to make himself seem far more important than he was," Trump said.
"It is the only thing he does well. Steve was rarely in a one-on-one meeting with me and only pretends to have had influence to fool a few people with no access and no clue, whom he helped write phony books," he added.
Bannon, executive chairman of right-wing news outlet Breitbart News, served as Trump campaign chief executive during the final months of the election. He is credited with helping to hone Trump's nationalism and economic populism as a candidate and president.
Bannon was forced out of the White House in August and eventually returned to Breitbart. He championed Republican Alabama Senate candidate Roy Moore, who defeated incumbent Sen. Luther Strange in the GOP primary before losing to now-Democratic Sen. Doug Jones in a special election in December.
Trump first supported Strange, then supported Moore over Jones.
In his statement, Trump said Bannon "had everything to do with the loss of a Senate seat in Alabama held for more than thirty years by Republicans."
Trump Jr. piled on in later tweets, mocking Bannon for Moore's loss and for the response to his back-and-forth with the president on the Breitbart website.
Bannon had heavily criticized the June 2016 meeting at Trump Tower, which the president's eldest son helped to set up. Attendees included Russian lawyer Natalia Veselnitskaya, Trump advisor and son-in-law Jared Kushner and campaign chairman Paul Manafort.
The meeting has figured in special counsel Robert Mueller's investigation into Russian efforts to influence the 2016 election and whether the Trump campaign colluded with Moscow.
Read Trump's full statement on Bannon here.