Mark Corallo was a spokesman for Marc Kasowitz, who is defending Mr Trump in an inquiry into alleged Russian meddling in last year's election.
Reports said that Mr Corallo disagreed with the alleged strategy of Mr Trump's lawyers to discredit or limit the team directing the investigation.
He confirmed his resignation on Friday.
Mr Corallo gave no further details about his decision to resign in his emailed statement.
However, a report published on the Politico website says that Mr Corallo is close to Justice Department special counsel Robert Mueller, who is leading the Russia investigation, and has previously praised him publicly.
Mr Corallo had grown frustrated with the operation of the legal team and the warring factions and lawyers, the report adds.
Mr Kasowitz, the New York lawyer who has represented Mr Trump's interests for more than a decade, will also see his role reduce, according to a CNN report, which cites sources with knowledge on the matter.
Instead, the report adds, defence attorney John Dowd and Jay Sekulow will now be Mr Trump's primary personal attorneys for the investigation.
Mr Mueller has hired big names to join his team, which is also investigating whether there was any collusion with the Trump team, which both Russia and Mr Trump have denied.
The New York Times reported that Mr Trump's team was looking to discredit the investigation led by Mr Mueller, analysing the background of the lawyers hired by him in search of any possible conflicts of interest.
Meanwhile, the Washington Post said that the president's lawyers were working to find ways to limit or undercut the investigation, also looking into possible conflicts of interest in Mr Mueller's team, and discussing the president's authority to grant pardons.