A multiplexer includes: one or more transmit filters that are connected between an antenna terminal and one or more transmit terminals; one or more receive filters that are connected between the antenna terminal and one or more receive terminals; and a circuit that includes a longitudinally coupled acoustic wave filter and is connected in parallel to one or more series resonators of a first filter that is one of the one or more transmit filters and the one or more receive filters and includes the one or more series resonators and one or more parallel resonators each formed of a piezoelectric thin film resonator, wherein the longitudinally coupled acoustic wave filter is formed of IDTs formed on a chip of a second filter that is another one of the one or more transmit filters and the one or more receive filters and formed of a surface acoustic wave resonator.