The paper reported that Michael Flynn, a retired three-star general who was among senior White House staff sworn in Sunday.
The paper reported that Michael Flynn, a retired three-star general who was among senior White House staff sworn in Sunday, has come under investigation as part of a counterintelligence examination of communications between Russian government members and Trump's inner circle.
Inquiry findings and whether it was still underway remained unclear, the WSJ said.
Flynn has raised eyebrows for receiving payment from the Russian broadcaster RT to join a gala celebration in Moscow last year, where he sat at a banquet table with President Vladimir Putin.
US reports have also said Flynn telephoned Moscow's ambassador to Washington, Sergey Kislyak, several times the day before Obama unveiled punitive measures over Russia's alleged cyberattacks to influence the US election.
Trump's press secretary Sean Spicer has said Flynn had spoken with the envoy on Christmas Day, sending him a text to wish him a merry Christmas and happy New Year.
The national security adviser is not formally part of the cabinet but is usually one of the president's most influential counsels.
Flynn, a veteran of America's wars in Iraq and Afghanistan, has courted controversy with extreme statements that critics say border on Islamophobia, but has taken a more flexible line on Russia and China.