Who was the first U.S. Heavyweight Boxing Champion to retire undefeated?

The first U.S. World Heavyweight Boxing Champion to retire undefeated was Rocky Marciano. He was born Rocco Francis Marchegiano on September 1, 1923, in Brockton, Massachusetts. During his career, he held the heavyweight boxing title for four years in the 1950s, and he is the only boxing champion to ever retire undefeated. His nickname was "Rocky" Marciano, and he fought as a professional for eight years. He retired on April 27, 1956. He was 32 years old. He finished his professional career with a record of 49–0.

Marciano was an American professional boxer who held the world heavyweight title from 1952 to 1956. He was able to go undefeated and to defend the championship title six times. He had championship fights against Jersey Joe Walcott, Roland La Starza, Ezzard Charles (twice), Don Cockell and Archie Moore.

Known for his relentless fighting style, punching power, stamina and iron chin, Marciano has been ranked by many boxing historians as one of the best heavyweight boxers of all time. On August 31, 1969, the day before his 46th birthday, he died in a private-plane crash near Des Moines, Iowa.

Regarding Joe Louis, he was a professional boxer who competed from 1934 to 1951. He reigned as the world heavyweight champion from 1937 to 1949. Jack Dempsey competed from 1914 to 1927, and reigned as the world heavyweight champion from 1919 to 1926. Floyd Patterson competed from 1952 to 1972, and twice reigned as the world heavyweight champion from 1956 to 1962.