Provided are a polarizer that resists red discoloration in a heat resistance test even when it has a small thickness, and a polarizing plate including the polarizer. The polarizer includes a polyvinyl alcohol-based resin layer and iodine adsorbed and oriented in the polyvinyl alcohol-based resin layer, and has a thickness less than or equal to 10 μm and a ratio of a retardation of iodine to a retardation of polyvinyl alcohol greater than or equal to 0.38.