In a substrate that improves isolation characteristics of a high-frequency-side signal path and a low-frequency-side signal path, a duplexer, and a substrate module, a package substrate includes two SAW filters mounted thereon and defines a portion of the duplexer. A substrate body includes main surfaces that oppose each other. Land electrodes are provided on one of the main surfaces and are used to connect either of the two SAW filters. Land electrodes are provided on one of the main surfaces and are used to connect a mounting substrate on which the duplexer is mounted and are respectively superposed with the land electrodes when viewed in plan from a z-axis direction. The land electrodes and the land electrodes, which are superposed with each other when viewed in plan from the z-axis direction, are electrically connected to each other.