Bannon will be a "tough and smart new voice @BreitbartNews ... maybe even better than ever before", CNN quoted Trump's tweets on Saturday.
On Friday, Bannon departed the White House after a turbulent seven-month tenure. Upon stepping down from his position, Bannon vowed to "go to war" with President Trump's opponents in the media and on the Capitol Hill.
Just hours after his ouster, Breitbart News announced that Bannon was back as executive chairman of the ultra-conservative website. The former adviser left Breitbart in 2016 to join Trump's presidential campaign.
Trump's tweet on Bannon was the second he posted on Saturday about his former chief strategist.
Sources told CNN that Bannon's ouster had been in the works for two weeks, and a source said that while he was given the option to resign, the former chief strategist was ultimately forced out.
Breitbart Editor-in-Chief Alex Marlow welcomed Bannon back in a statement, saying that the website had "gained an executive chairman with his finger on the pulse of the Trump agenda".
However, the mood inside Breitbart was mixed.
One person familiar with the matter told CNN that there was a contingent of Breitbart staffers who desperately wanted Bannon to return.
But others had hoped Bannon would not return, another source said, citing the brash and bombastic manner in which he worked with employees.
Bannon is the latest high-profile figure to be removed from the White House team after the ouster of Communications Director Anthony Scaramucci, Chief of Staff Reince Priebus, Press Secretary Sean Spicer and National Security Adviser Michael Flynn.