Midfielder Neri Cardozo scored the only goal of the game with a left-foot shot in the 37th minute.
Boca was reduced to 10 men in the second half when Fabian Vargas was sent off in the 65th minute for a high tackle. Vargas will miss the final.
Etoile had several chances to even the match, but could not capitalize. Boca will face the winner of Thursday's other semifinal between European champions AC Milan of Italy and the Urawa Reds of Japan.
Boca beat Milan on penalty kicks to win this tournament in 2003 in the former Toyota Cup, a one-off competition between the European and South American club champions. In 2000, Boca beat Real Madrid, 2-1. The Club World Cup features the club champions from the International Football Federation's continental tournaments.