William Marino Guerra (born 24 February 1968) is a former footballer who played international football for San Marino as a defender. After graduating from the country's youth set-up, he made forty appearances for the San Marino national football team, two in 1987 before the country was affiliated to FIFA and thirty-eight FIFA-sanctioned games between 1990 and 1999, making him the nation's tenth most-capped player of all time. Guerra also captained San Marino on five occasions between 1995 and 1997. He split a 16-year club career between teams from Italy and San Marino.