Provided are a composition for forming a polyurethane resin used for a membrane sealing material, the composition having low viscosity and contributing to the formation of a membrane sealing material having reduced elution of low-molecular weight reaction products; and a membrane sealing material and a membrane module using this composition. Disclosed is a composition for forming a polyurethane resin used for a membrane sealing material, the composition including a base agent (A) and a curing agent (B), wherein the base agent (A) comprises an isocyanate group-terminated prepolymer (A1), the isocyanate group-terminated prepolymer (A1) is a reaction product between an isocyanate group-containing compound (a1) and an active hydrogen-containing compound (a2), the base agent (A) comprises diphenylmethane diisocyanate, the monomer content of the diphenylmethane diisocyanate is 35% by mass or less, and the curing agent (B) comprises a compound (b1) represented by Formula (1). In Formula (1), R represents an alkyl group having 2 or more carbon atoms.