Sergio Bismarck Villar (born 5 January 1944) is a Uruguayan former footballer who played as a left back. He played most of his career for San Lorenzo de Almagro in Argentina. Villar started his career in Uruguay where he played for Club Sportivo Cerrito, C.A. Canillitas and Defensor S.C. In 1968 Villar joined San Lorenzo and in his first year with the club they won the Metropolitano championship, and became the first team in the professional era of Argentine football to complete a season without losing a game. Villar had a short spell with All Boys in 1982, after which he retired.