A federal judge ruled on Wednesday that President Trump broke the law when he fired members of an independent civil liberties watchdog without cause in January, ordering the reinstatement of those challenging their removals in court.

In a 71-page opinion, Judge Reggie B. Walton of the Federal District Court for the District of Columbia said that Mr. Trump’s dismissal of Democratic-selected members to the five-person Privacy and Civil Liberties Oversight Board was illegal.

Lawmakers, the judge wrote, clearly intended to shield board members from arbitrary removal by a president before their terms were up. He noted that Congress created the bipartisan panel to oversee government counterterrorism actions and policies on the recommendation of the commission that investigated the Sept. 11, 2001, terrorist attacks.

“That responsibility is incompatible with at-will removal by the president, because such unfettered authority would make the board and its members beholden to the very authority it is supposed to oversee on behalf of Congress and the American people,” wrote Judge Walton, an appointee of President George W. Bush.

The judge issued a permanent injunction ordering the government to let the two board members challenging their dismissals, Travis LeBlanc and Edward W. Felten, immediately return to work. In a statement, Mr. LeBlanc celebrated the outcome.