John Leroy Christiansen(born December 20, 1928, in Sublette, Kansas; died June 29, 1986 at age 57)was a professional football player for the Detroit Lions.

Christiansen played football at Colorado State, and he was drafted by the Lions in 1951, and enjoyed success in his rookie year. In one crucial contest against the Los Angeles Rams as a rookie, Christiansen scored twice on punt returns of 69 and 47 yards. He also scored on two punt returns (71 and 89 yards) against the Green Bay Packers that season.

Jack's forte was defense and he was the key man for the defensively-strong Lions, who in the 1950s dominated the National Football League with four divisional and three world championships. He led the league in interceptions in 1953 and tied for the league lead in 1957.

Christiansen played eight seasons with the Lions from 1951-58 and was named All-Pro six consecutive years (1952-1957) and played in five consecutive Pro Bowls (1954-1958).