A polishing composition of the present invention is to be used for polishing an object including a portion containing a group III-V compound material. The polishing composition contains abrasive grains, an oxidizing agent, and a water-soluble polymer. When the polishing composition is left to stand for one day in an environment with a temperature of 25° C., the water-soluble polymer may be adsorbed on the abrasive grains at 5,000 or more molecules per 1 μm2 of the surface area of the abrasive grains. Alternatively, the water-soluble polymer may be a compound that reduces the water contact angle of the portion containing a group III-V compound material of the object after being polished with the polishing composition.