A thermoplastic polyester resin composition includes 0.05 to 10 parts by weight of an epoxy compound (B) and 0.001 to 1 part by weight of a hindered amine compound (C) with respect to 100 parts by weight of a thermoplastic polyester resin (A) having an amount of carboxyl groups of 50 eq/t or less. The thermoplastic polyester resin composition is capable of producing a molded article which is excellent in mechanical properties and heat resistance, as well as in long-term hydrolysis resistance, and has a small decrease in mechanical properties and hydrolysis resistance even when melt-processed at a high temperature of 270° C. or more.