Trump, who has been silent about the transaction for months, told a friend in the last few weeks that he still considers the merger to be a bad deal, Bloomberg reported. (bloom.bg/2jfkJlx)
He believed the deal would concentrate too much power in the media industry, Bloomberg said.
Shares of AT&T were down 0.2 percent, while Time Warner's stock was down 1.6 percent.
Trump's chief strategist, Steve Bannon, is also opposed to the deal, Bloomberg reported, citing another person.
Trump during his campaign had said AT&T's proposal to buy the owner of CNN and the Warner Bros movie studio was an example of a "power structure" that was rigged against him and voters.
Trump's transition team, AT&T and Time Warner did not immediately respond to requests for comment.
Reporting by Anya George Tharakan in Bengaluru; Editing by Sriraj Kalluvila