A semiconductor amplifier includes a semiconductor amplifying element, an output terminal, an output matching circuit, and an output bias circuit. The output matching circuit includes a bonding wire, a first transmission line, and a second transmission line. The other end part of the first transmission line is connected to one end part of the second transmission line. The output bias circuit includes a third transmission line having an electrical length of approximately 90° at a center frequency, a grounded shunt capacitor, and a power supply terminal. The third transmission line includes one end part and the other end part connected to the grounded shunt capacitor. The one end part of the third transmission line is connected to the second transmission line at a position where an electrical length is approximately 45° from the one end part of the second transmission line at the center frequency.