Philip Benjamin Heymann (October 30, 1932 – November 30, 2021) was an American legal scholar and federal prosecutor who headed the Criminal Division of the Justice Department as Assistant Attorney General during the Carter administration and was briefly Deputy Attorney General in the Clinton administration before he resigned over management and policy differences as well as perceived interference by the White House. He was involved internationally in supporting the rule of law in criminal justice systems. In domestic politics he was a vocal supporter of civil and political liberties and, as such, was actively critical of the George W. Bush administration, particularly its warrantless domestic spying program. Even before the September 11 attacks, Heymann studied and published on how prosecution of antiterror policies can be done consistent with the rule of law in a democratic society. He was later James Bar Ames Professor of Law, Emeritus at Harvard Law School, where he began teaching in 1969.

Heymann was born in the Squirrel Hill section of Pittsburgh on October 30, 1932, as one of two children of Sidney B. and Bessie (Kann) Heymann. His father owned an insurance agency. He was a 1950 graduate of Pittsburgh's Shady Side Academy. Heymann's sister, Sidney (known as Sally) became a licensed psychologist in Washington, Pennsylvania. She died in 1991.

In 1954 he received his B.A. degree summa cum laude from Yale University, where he was a member of Scroll and Key Society. He was awarded a Fulbright grant, and he studied at the Sorbonne in Paris for the 1954–1955 academic year. He then served two years in the Air Force's Office of Special Investigations, reviewing security clearances. He later received his J.D. degree from Harvard Law School, where he was third in his class and one of two case editors of the Harvard Law Review. Thereafter, he clerked for Justice John M. Harlan during the 1960-1961 term.

From 1961 to 1965 Heymann practiced in the office of the Solicitor General of the United States under Archibald Cox. After his admission to the Supreme Court bar in 1963, Heymann argued six cases before the Supreme Court. Only one of these, however, received any publicity, and that was owing to the unusual levity of the court on that occasion. Heymann left the Solicitor General's office shortly after President Johnson accepted Cox's pro forma resignation at the end of the court's term in 1965.

In September 1965 he became deputy in the Bureau of Security and Consular Affairs of the State Department and was appointed acting administrator in March 1966. Right before his appointment he issued a reprimand to the head of the Passport Office for asking the U.S. embassies in Paris and Moscow to report on the activities of Harvard history professor and anti-nuclear activist H. Stuart Hughes. The written reprimand cited the significant freedom that Americans ought to enjoy in freedom of movement. After issuing the reprimand, the Bureau learned that the FBI had asked for the instructions and that such surveillance requests had routinely been made without the knowledge of the directors of the Bureau for at least two decades. The affair became something of a political issue, an early pushback against government surveillance. After serving as acting administrator of the Bureau for nearly a year, Heymann was appointed Deputy Assistant Secretary in the State Department's Bureau of Internal Organizations. In 1967 he became Executive Assistant to Under Secretary of State Nicholas Katzenbach. In 1968 Heymann helped Katzenbach force a reluctant State Department bureaucracy to finally undertake a review of the denial of security clearance of John Paton Davies, one of the China Hands whose security clearance was revoked 14 years before by then Secretary of State John Foster Dulles, who made the decision under pressure of Joseph McCarthy's allegations. The conduct of Davies was vindicated in the later review and his security clearance re-instated.

Heymann left the State Department for Harvard Law School where he taught as visiting professor from July 1969. He was one of the very few faculty hires by the law school who had a substantial amount of non-academic experience between graduation and appointment. Heymann also became a faculty member at Harvard Kennedy School.

Heymann believed that law students were better served by being taught how to build institutions rather than merely instructed in legal ideas. At the beginning of his academic career Heymann worked to introduce law students to some of the methods taught in the Kennedy School for creating and managing institutions. In 1979 he proposed and the Law School tentatively approved a proposal for a 13-hour elective in such fields as analytical methods, economic theory, statistical methods and political theory.

At Harvard he was now a colleague of Archibald Cox, his boss in the Solicitor General's office. Their political impulses were roughly the same. Heymann joined Cox and most of the Law School faculty, for instance, in an open letter to Congress, urging an end to the Vietnam war at the height of student unrest following the Kent State shootings. Cox, unlike Heymann, however, refused to involve himself in the politics of the Supreme Court. Heymann collaborated with Cox in drafting the amicus curiae brief to the Massachusetts Supreme Judicial Court on behalf of the American Mutual Assurance Alliance and the American Insurance Association, in support of the constitutionality of Massachusetts's No-fault insurance scheme, a piece of legislation with a decided Harvard connection.