An outdoor unit of an air-conditioning apparatus has a structure that enables a terminal block to be added on a front side that a worker can easily contact, without greatly changing the location of an existing terminal block and the basic shape of a mounting portion. In a casing of the outdoor unit, a heat exchanger, an outdoor fan, a compressor, and an electrical component are provided. The outdoor unit includes: terminal blocks to be connected to the electrical component; and a terminal-block mounting portion facing from the inside of the casing toward the outside thereof. The terminal blocks are arranged in the height direction of the terminal-block mounting portion, and include first and second terminal blocks on a lower side and an upper side, respectively. The second terminal block is mounted on a terminal-block seat on the terminal-block mounting portion. The above seat includes a mounting portion on which the second terminal block is mounted, and a fixed portion contacting the terminal-block mounting portion. The fixed portion is smaller than the mounting portion, and the mounting portion protrudes from the fixed portion toward the first terminal block.