Trump has threatened to sue the New York Times among other outlets numerous times but he has never followed through on any of those threats because a lawsuit would involve a discovery process that wouldn't turn out well for him.
But Trump himself is not suing the New York Times.
His campaign is. The Trump campaign filed a lawsuit today at the New York State Supreme Court and they're accusing the Times of publishing something they never actually published.
“Today the President’s re-election campaign filed suit against the New York Times for falsely stating the Campaign had an ‘overarching deal’ with ‘Vladimir Putin’s oligarchy’ to ‘help the campaign against Hillary Clinton’ in exchange for ‘a new pro-Russian foreign policy, starting with relief from ... economic sanctions,’” said Jenna Ellis, senior legal to Donald J. Trump for President, Inc.
“The statements were and are 100 percent false and defamatory. The complaint alleges The Times was aware of the falsity at the time it published them, but did so for the intentional purpose of hurting the campaign, while misleading its own readers in the process,” Ellis said.
Neither the New York Times or anyone else has ever reported that the Trump campaign had a direct, legitimate "deal" with Vladimir Putin even if the facts make it appear as though they did.
Of course, if we're being real, the truth of the matter is not really up for debate. The goal of this lawsuit is not to disprove something that never happened; it's to attack the media and New York's "liberal" courts.
I'm not a lawyer, but one of my first questions when the news broke was whether or not the current Trump campaign even has legal standing to file this lawsuit. But even if the case is thrown out, Trump will use that, too.