San Marcos was held to two hits by Buena lefty Trevor Hammond and suffered a 7-0 loss in its final game in the Southern California Invitational baseball tournament.
The Royals finished 2-2 in the tournament.
Jack Winterbauer and Dominic Roderick had the hits for the Royals (2-2).
Hammond pitched a complete game and struck out nine.
Isaac Villarreal suffered the loss for San Marcos.
San Marcos plays at Cabrillo in a Channel League opener on Friday at 3:30 p.m.