To provide an N-alkylmaleimide-based polymer particle that contains a small amount of water-soluble celluloses exerting a bad influence on the storage stability of a polymer solution, and a method of producing the N-alkylmaleimide-based polymer particle.

The N-alkylmaleimide-based polymer particle includes an N-alkylmaleimide residue expressed by general formula (1) and contains 150 ppm or less of the water-soluble celluloses, and the method produces the N-alkylmaleimide-based polymer particle (in general formula, R₁ represents 1C-12C linear alkyl, 3C-12C branched alkyl, or 3C-6C cyclic alkyl).