An example of a high-frequency semiconductor device includes two unit semiconductor devices. Each of the two unit semiconductor devices has a ground substrate, a high-frequency semiconductor element, an input-side matching circuit, an output-side matching circuit, a side wall member, an input terminal, and an output terminal. The ground substrate has heat-radiating property. The high-frequency semiconductor element is provided on the ground substrate. The input-side matching circuit is connected to the high-frequency semiconductor element. The output-side matching circuit is connected to the high-frequency semiconductor element. The side wall member surrounds at least the high-frequency semiconductor element. The input terminal is connected to the input-side matching circuit. The output terminal is connected to the output-side matching circuit. The two unit semiconductor devices are coupled to each other at upper edges of the side wall members.