Provided are: a resin modifier which is capable of imparting a resin with excellent moisture resistance, dimensional stability and optical properties; and a resin composition including the same. The resin modifier contains a compound represented by the following Formula (1): (wherein, m represents an integer of 1 to 5; n represents an integer of 0 to 50; and RA, RB and RC each represent a hydrocarbon group having 2 to 20 carbon atoms, or a hydrocarbon group having 2 to 20 carbon atoms which has at least one atom selected from the group consisting of an oxygen atom, a sulfur atom and a nitrogen atom).