Speaking to reporters outside the White House on Tuesday, Mr Trump said his differences with Mr Tillerson came down to personal “Chemistry”.
Mr Tillerson is the latest in a long list of senior officials who have either resigned or been fired since Mr Trump took office.
The Department of State said Mr Tillerson had not spoken to the president and was “Unaware of the reason” for his firing.
Mr Tillerson said later in a news conference that he had received a call from the president on Air Force One after midday, about three hours after Mr Trump fired him via tweet.
The secretary of state was on an official tour of Africa last week when he was caught unawares by Mr Trump’s agreement to meet North Korean leader Kim Jong-un.
Mr Tillerson was reported to be astonished at how little Mr Trump grasped the basics of foreign policy.