Mark Corallo, who coordinated the Trump legal team's public response to the crisis, told AFP in an email that he had resigned from his post.
No reason was given, but the move comes after Trump waded into potentially perilous legal territory by warning investigators not to look into his family finances.
In an expansive interview with the New York Times earlier this week, Trump appeared to make that a red line for special counsel Robert Mueller.
Mueller is examining whether Trump or his aides colluded with Russia's efforts to influence the 2016 presidential election.
Trump has repeatedly denied any wrong doing, but has struggled to explain why his eldest son and key aides met Russian operatives who promised dirt on Hillary Clinton.
With the investigation apparently extending to financial transactions, US media reported that Trump allies were looking into issuing presidential pardons and for ways to discredit Mueller's investigation.
Trump himself has suggested that Mueller -- a widely respected former FBI director -- may have a conflict of interest.
"There is NO basis to question the integrity of Mueller or those serving with him in the special counsel's office," said former attorney general Eric Holder.
"Trump cannot define or constrain Mueller investigation. If he tries to do so this creates issues of constitutional and criminal dimension."
The White House has pointedly refused to rule that out the possibility that Trump would fire Mueller -- an act that would prompt a political firestorm and perhaps a constitutional crisis.
Trump has already fired his FBI director James Comey over the Russia investigation and lashed out at his own attorney general for recusing himself from the probe.
The lead Democrat on a House of Representative's panel that is separately investigating Russian actions around the time of the election also warned Trump was wandering into dangerous territory.
"There is no doubt that Mueller has the authority to investigate anything that arises from his investigation into the Trump campaign's ties to Russia, including financial links," said Representative Adam Schiff.
The top Democrat in the Senate's investigation warned that pardoning anybody who may have been involved "would be crossing a fundamental line."
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)