A multiplexer includes a filter (10) arranged between a common terminal and an input/output terminal (110) and configured to pass a radio frequency signal in a first frequency band, and a filter (20) arranged between the common terminal and an input/output terminal (120) and configured to pass a radio frequency signal in a second frequency band. The filter includes series arm circuits (31 and 32) connected in series, a series arm circuit (33) connected in parallel to the series arm circuit (32), and a parallel arm circuit. The series arm circuit (32) includes a series arm resonator that is an acoustic wave resonator. The series arm circuit (33) includes a switch arranged on a second path connecting nodes. In a CA mode, the switch is OFF. In a non-CA mode, the switch is ON.