Allen McIntyre Stack (January 23, 1928 – September 12, 1999) was an American competition swimmer, Olympic champion, and former world record-holder.

Stack won the gold medal in the men's 100-meter backstroke at the 1948 Summer Olympics in London.

[1] Stack attended Yale University, where he swam for the Yale Bulldogs swimming and diving team in National Collegiate Athletic Association (NCAA) competition from 1947 to 1949.

He served in the U.S. Navy from 1951 to 1954, and graduated from Columbia University Law School in 1956.

[2] Stack was inducted into the International Swimming Hall of Fame as an "Honor Swimmer" in 1979.