The present invention provides a thermoplastic polyester elastomer composition which is flexible and exhibits excellent thermal aging resistance and water resistance. According to the present invention, there is provided a thermoplastic polyester elastomer composition wherein, to 100 parts by mass in total of a thermoplastic polyester elastomer (A) and a modified hydrogenated styrene-type elastomer (B), 0.1 to 10 part (s) by mass of a carbodiimide compound (C), 0.01 to 5 part (s) by mass of an antioxidant of a hindered phenol type (D) and 0.01 to 5 part (s) by mass of an antioxidant of a sulfur type (E) are contained, wherein the thermoplastic polyester elastomer (A) comprises a hard segment comprising a polyester constituted from an aromatic dicarboxylic acid and an aliphatic diol or an alicyclic diol and a soft segment comprising an aliphatic polyether as main constituent ingredients, wherein a content of the soft segment is 3 to 40% by mass, wherein a rate by mass of the above (A) to the above (B) ((A)/(B)) is from 95/5 to 40/60.