The attorneys and researchers say he is receiving payments from foreign governments via guests at his hotels and leases on his buildings.
They argue that a clause in the constitution bans such payments.
Mr Trump described the lawsuit as "totally without merit".
His son, Eric Trump, who is an executive vice-president of the Trump Organization, said the company had taken greater steps than the law required to avoid legal exposure, the New York Times reported.
He said it had agreed to donate any hotel profits that come from foreign government guests to the US treasury.
Citizens for Responsibility and Ethics in Washington (Crew) filed the lawsuit in a Manhattan federal court on Monday morning.
"We did not want to get to this point," Crew executive director Noah Bookbinder said in a statement.
"It was our hope that President Trump would take the necessary steps to avoid violating the Constitution before he took office. We were forced to take legal action."
The US constitution says no federal official should receive a gift or "emolument" - a fee - from a foreign government.
"President Trump has made his slogan America First so you would think he would want to strictly follow the constitution's foreign emoluments clause, since it was written to ensure our government officials are thinking of Americans first, and not foreign governments," Mr Bookbinder said.
Mr Trump's lawyers argue that the clause is intended only to stop federal officials from accepting a special consideration or gift from a foreign power and does not apply to payments such as a bill for a hotel room.
A group of US ethics lawyers have filed a lawsuit against President Donald Trump, alleging he is violating a constitutional ban on accepting payments from foreign governments.