An object of the present invention is to provide a polarizing element which has an excellent antireflection function as being applied to an image display device; and a circularly polarizing plate and an image display device, each of which has the polarizing element. The polarizing element of an embodiment of the present invention has an alignment film and an anisotropic light-absorbing film formed using a dichroic substance, in which a degree S of alignment of the anisotropic light-absorbing film is 0.92 or more, and an average refractive index nave at a wavelength of 400 to 700 nm of the alignment film is 1.55 to 2.0.