President Trump must face a defamation lawsuit by a former contestant on his reality TV show “The Apprentice” after a Manhattan judge ruled in a first-of-its-kind decision that he could not claim immunity through his job as the nation’s commander-in-chief.

“No one is above the law,” Justice Jennifer Schecter wrote in the 19-page decision released Tuesday.

Trump had argued that presidents are shielded from civil litigation in state courts under the US Constitution’s supremacy clause, but that assertion had never been fully tested by the courts — until now.

“It is settled that the President of the United States has no immunity and is ‘subject to the laws’ for purely private acts,” Schecter added.

The decision means that Summer Zervos, who in 2016 accused then-candidate Trump of groping her and pressing his privates against her in 2007, can pursue her defamation case against the president for saying that her allegations were “fiction” and claiming she made them up for “personal gain.”

His comments could lead people to view the former reality TV star as “contemptible,” the judge wrote.

The ruling also means that Trump will likely have to sit for a deposition in the case as Schecter said the suit has merit.

“His statements can be proven true or false, as they pertain to whether [Zervos] made up allegations to pursue her own agenda,” the judge said.

Trump’s attorney Marc Kasowitz had asked the judge to toss or delay the case until the president left office, arguing that “there can’t be any curtailment” on his duties.

“Thomas Jefferson made clear that the president’s responsibilities are 24/7, every single day no matter what he or she is doing,” Kasowitz said during oral arguments in December.

But Schecter said there’s “absolutely no authority for dismissing or staying the case.”

“In Clinton v Jones the United States Supreme Court held that a sitting president is not immune from being sued in federal court for unofficial acts,” Schecter wrote, citing the sexual harassment suit that led to the 1998 impeachment of President Bill Clinton for lying under oath about his affair with White House intern Monica Lewinsky.

“It left open the question of whether concerns of federalism and comity compel a different conclusion for suits brought in state court. Because they do not, defendant’s motion to dismiss this case or hold it in abeyance is denied,” Schecter ruled.

“Nothing in the Supremacy Clause of the United States Constitution even suggests that the president cannot be called to account before a state court for wrongful conduct that bears no relationship to any federal executive responsibility,” Schecter found.

“Significantly, when unofficial conduct is at issue, there is no risk that a state will improperly encroach on powers given to the federal government by interfering with the manner in which the President performs federal functions,” she wrote.

But the judge said that “important federal responsibilities will take precedence” over the civil case.

Still, she said, “There are no compelling reasons for delaying plaintiff’s day in court.”