A method of producing modified vinyl alcohol-based polymer particles, includes mixing vinyl alcohol-based polymer particles with a mixed medium containing unsaturated carboxylic acid, an acid catalyst, and water, wherein, by mixing the vinyl alcohol-based polymer particles with the mixed medium, the vinyl alcohol-based polymer particles are reacted with the unsaturated carboxylic acid to obtain modified vinyl alcohol-based polymer particles containing a vinyl ester unit represented by a formula (2) below, having a vinyl alcohol unit content based on the total constitutional units of 60 mol % or more and less than 95 mol %, and having an average particle diameter from 50 to 2000 μm. The production method is capable of controlling the particle shape and does not cause the problem of the residual sulfur content, and is further convenient and economically advantageous.