Leonard I. Garth (born April 7, 1921) is a senior judge on the United States Court of Appeals for the Third Circuit.

Garth was born in Brooklyn, New York, and graduated with a bachelor of arts from Columbia University in 1942. He served during World War II as a United States Army Lieutenant from 1943 to 1946. Upon his return, he earned his law degree from Harvard Law School in 1952, and built a private practice in Paterson, New Jersey

On July 22, 1969, Garth was nominated by President Richard M. Nixon to a seat on the U.S. District Court for the District of New Jersey vacated by Thomas M. Madden. Garth was confirmed by the United States Senate on December 17, 1969, and received his commission on December 18, 1969. On July 19, 1973 Nixon nominated Garth for elevation to a seat on the United States Court of Appeals for the Third Circuit vacated by James Rosen. Garth was confirmed by the United States Senate on August 3, 1973, and received his commission on August 6, 1973. He assumed senior status on June 30, 1986.

Supreme Court Justice Samuel Alito clerked for Garth from 1976 to 1977 in his first job out of law school. As of 2013 Garth holds the distinction of being the only sitting judge for whom a member of the United States Supreme Court has clerked. Garth had been a lecturer at Rutgers Law School and at Seton Hall Law School.