A resin composition containing: (A) one or more resins selected from the group consisting of a thermoplastic resin, and a curable resin selected from an epoxy resin, a (meth)acrylic resin, a phenolic resin, an unsaturated polyester resin, a polyurethane resin, or a polyimide resin; and (B) modified cellulose fibers wherein one or more substituents selected from substituents represented by the following general formulas (1) and (2): —CH2—CH(OH)—R1 (1), —CH2—CH(OH)—CH2—(OA)n-O—R1 (2), wherein each R1 in the general formulas (1) and (2) is independently a linear or branched alkyl group having 3 or more carbon atoms and 30 or less carbon atoms; n in the general formula (2) is a number of 0 or more and 50 or less; and A is a linear or branched, divalent saturated hydrocarbon group having 1 or more carbon atoms and 6 or less carbon atoms are bonded to cellulose fibers via an ether bond, wherein the modified cellulose fibers have a cellulose I crystal structure. The resin composition of the present invention can be suitably used in various applications such as daily sundries, household electric appliance parts, wrapping materials for household electric appliance parts, automobile parts, and resins for three-dimensional modeling.