Yes, a sitting president can be subjected to such a lawsuit for unofficial acts, according to the U.S. Supreme Court in its 1997 decision Clinton v. Jones. In 1982, the Court had ruled in Nixon v. Fitzgerald that a president enjoys absolute immunity from damages based on official acts.

The suit in question was a sexual harassment complaint filed by former Arkansas state employee Paula Jones against President Bill Clinton. Jones alleged that Clinton, at the time the governor of Arkansas, propositioned her for sexual favors in a Little Rock hotel room. She filed a sexual harassment claim in federal court. Attorneys for the president argued that he should enjoy immunity from the lawsuit while he occupied the White House and performed his duties as president.

The U.S. Supreme Court determined that Jones’s lawsuit could proceed in part because the conduct complained of was not an official act of the president. The conduct occurred before he became president. President Clinton argued that separation of powers principles supported his contention that the lawsuit should be delayed until after he left office. The Court unanimously rejected that argument, writing that “the doctrine of separation of powers does not require federal courts to stay all private actions against the President until he leaves office.”

The case proceeded against President Clinton after the Court’s decision. He gave a deposition in January 1998 at the Washington, D.C., office of his lawyer, Robert Bennett. In April 1998, a federal district court judge dismissed the lawsuit. Jones appealed this dismissal to the Eighth U.S. Circuit Court of Appeals, which heard arguments in the case in October 1998. Clinton settled with Jones by agreeing to pay her $850,000, though he admitted no wrongdoing.