Quoting unidentified sources, the Journal said guards from the royal court detained one of the king's brothers, Prince Ahmed bin Abdulaziz al Saud, and one of his nephews, Prince Mohammed bin Nayef. Both men were arrested at their homes and charged with treason, it said.
There was no immediate comment from Saudi authorities on the report.
Mohammed bin Nayef, a once powerful figure as head of Saudi counterterrorism efforts, had been crown prince until 2017, when King Salman took away the title and put his son first in line for the throne of the longtime U.S. ally.
The crown prince, who is in charge of day-to-day governance in the kingdom, has been praised in the West for implementing social reforms, but he also has drawn intense criticism for a tough crackdown on Saudis perceived as critics of his policies.
He also came under criticism after the 2018 killing of Saudi dissident and Washington Post columnist Jamal Khashoggi inside the Saudi consulate in Istanbul. Critics accused him of being linked to the slaying, but he denied it. A Saudi court sentenced five people to death for the killing, but did not hold any high-ranking officials responsible.
Congress has also harshly criticized Saudi Arabia for its war in Yemen, which has led to the world's worst humanitarian disaster.