A source close to Bannon told ABC News the resignation was effective on Aug. 14, exactly one year after he joined the Trump campaign.
On Tuesday, President Donald Trump told reporters at Trump Tower that Bannon was a "good man” and “not a racist.”
"I like Mr. Bannon. He's a friend of mine. But Mr. Bannon came on very late. You know that. I went through 17 senators, governors, and I won all the primaries. Mr. Bannon came on very much later than that," Trump said before adding, "but we'll see what happens with Mr. Bannon."
Bannon has clashed with virtually every top official in the White House. Atop his list of in-house detractors are senior adviser Jared Kushner, national security adviser Lt. Gen. H.R. McMaster and the new chief of staff, Gen. John Kelly.
The former executive chairman of Breitbart News joined the Trump campaign last August. He would become known as a fearless and critically influential adviser to the president.
Bannon is the latest high-profile aide to leave the White House. On July 21, press secretary Sean Spicer resigned, followed by Chief of Staff Reince Priebus. White House communications director Anthony Scaramucci was fired a few days later, serving just 11 days in that role.