A rheology control agent for a curable composition may include: a diamide compound (A) and/or a hydrogenated castor oil (A′), the diamide compound (A) being obtained by condensation reaction between a diamine component (A1) and a monocarboxylic acid component (A2); and a polyamide compound (B) obtained by polycondensation of an amine component (B1) and a carboxylic acid component (B2). The diamine component (A1) may be selected from the group consisting of diamines with 2 to 12 carbon atoms. The monocarboxylic acid component (A2) may be selected from the group consisting of hydrogenated castor oil fatty acids and linear saturated fatty acids. The polyamide compound (B) may have a weight-average molecular weight from 2,000 to 21,000. A cured product of the curable composition may be used for a sealant or an adhesive.