The president of the United States enjoys absolute immunity from many lawsuits while in office; it is legally untested whether they also enjoy criminal immunity from arrest or prosecution. Neither civil nor criminal immunity is explicitly granted in the Constitution or any federal statute.
The Supreme Court of the United States found in Nixon v. Fitzgerald (1982) that the president has absolute immunity from civil damages actions regarding conduct within the "outer perimeter" of their duties. However, in Clinton v. Jones (1997), the court ruled against temporary immunity for sitting presidents from suits arising from pre-presidency conduct. Some scholars have suggested an immunity from arrest and criminal prosecution as well, a view which has become the practice of the Department of Justice under a pair of memoranda (1973 and 2000) from the Office of Legal Counsel. Presidents Richard Nixon, Bill Clinton, and Donald Trump were criminally investigated while in office, but none was prosecuted while in office. No court has ever ruled on the matter of criminal immunity.
Civil immunity
Background
The Constitution of the United States grants legislative immunity to members of Congress through the Speech or Debate Clause, but has no explicit comparable grant for the president. Early American politicians, including those at the Constitutional Convention, were divided as to whether such immunity should exist. However, courts historically found that the president had absolute immunity from any personal damage liability for acts undertaken in the course of their duties. The first suit brought directly against a president was Mississippi v. Johnson (1867), in which the Supreme Court of the United States ruled Andrew Johnson could not be sued as the actions in question were discretionary. Spalding v. Vilas (1896) affirmed that federal cabinet officers had absolute immunity for actions "more or less" within the scope of their duties; Barr v. Matteo (1959) extended this to all federal executive officials.
Suits against Nixon
No court was willing to assert jurisdiction over the president until the D.C. District Court did so over Richard Nixon in Minnesota Chippewa Tribe v. Carlucci (1973). After the D.C. Circuit Court of Appeals entered a declaratory judgment against Nixon in National Treasury Employees Union v. Nixon (1974) and contemplated the possibility of a writ of mandamus against him, a wave of suits directly against Nixon began. In 1978, in Butz v. Economou, the U.S. Supreme Court held that in a constitutional cause of action (as allowed in Bivens v. Six Unknown Named Agents [1971]), Spalding and Barr (which were about common law causes of action) did not control, and federal executive officials were entitled only to qualified immunity, not absolute. The next year in Halperin v. Kissinger, the D.C. Circuit extended that logic to Nixon, who had by then resigned.
In 1978, whistleblower A. Ernest Fitzgerald added former president Nixon to his suit against several officials involved in his firing from the Department of the Air Force. This resulted in the collateral appeal Nixon v. Fitzgerald (1982), in which the U.S. Supreme Court ruled that a former or current president was absolutely immune from suit regarding acts within the "outer perimeter" of his duties, citing the president's "unique status under the Constitution". A four-justice dissent objected to a scope that included willful violations of the Constitution and would have given immunity only to certain functions of the presidency.
Clinton v. Jones
Paula Jones sued Bill Clinton in 1994 for several counts related to allegedly sexually harassing her when he was governor of Arkansas. Clinton, by then the president, sought both to dismiss the case with prejudice on the basis of immunity and to toll the statute of limitations for the duration of his presidency. The court declined to dismiss, but stayed the trial until Clinton's presidency ended. The Eighth Circuit affirmed, and in Clinton v. Jones the U.S. Supreme Court in turn affirmed the Eighth Circuit, holding that presidential immunity generally does not extend to lawsuits over matters that predate the president taking office.
Criminal immunity
Constitutional Provisions
Article II, Section 4 provides for which crimes the President shall be removed from office by impeachment in the House and conviction in the Senate. Article I, Section 3, Clause 7 specifies that a President impeached by the House and convicted by the Senate is nevertheless subject to indictment, trial, judgement, and punishment "by law".