The present invention addresses the problem of providing a polarizer maintaining an excellent degree of polarization and having high transmittance and excellent durability, and a polarizing plate and an image display device using the same. This polarizer has a polyvinyl alcohol-based resin and iodine included in the polyvinyl alcohol-based resin, in which the thickness of the polarizer is 2 to 20 μm, the degree of orientation of the polyvinyl alcohol-based resin is 0.11 or more and 0.16 or less, the iodine content is more than 0.50 g/m2 and 1.0 g/m2 or less, and the product of the degree of orientation of the polyvinyl alcohol-based resin and the iodine content is 0.08 g/m2 or more and 0.11 g/m2 or less.