A new species of the genus Isocolus Förster, 1869 (Hymenoptera, Cynipidae: Aylacini) is described. It induces galls on the capitula of Centaurea ornata Willd. (Asteraceae), an endemic of the Iberian Peninsula. Isocolus melikai Pujade-Villar n. sp. is morphologically closely related to I. freidbergi Melika, 2008, forming gall on C. verutum L., 1755, and endemic to the North West of the Arabian Peninsula. Urophora cuspidata (Meigen, 1826) (Diptera, Tephritidae) has also been obtained from similar galls on C. ornata. Isocolus melikai Pujade-Villar n. sp. was found in 16 of the 326 inflorescences (capitula) sampled, while both species (I. melikai Pujade-Villar n. sp. and U. cuspidata) were found together only in three of these capitula. The galls are also described and a probable parasitoid of Ormyrus Westwood, 1832 (Hymenoptera, Ormyridae) is mentioned. The most important characteristics to differentiate adults and galls are illustrated. Biological data are provided showing that I. melikai Pujade-Villar n. sp. is bivoltine.