The genus Rhaponticoides, recently segregated from the genus Centaurea, belongs to the family Asteraceae (tribe Cardueae). Rhaponticoides mykalea (Hub.-Mor.) M.V.Agab. & Greuter is an endemic species narrowly distributed in the Aegean part of Turkey. Owing to the existing anthropogenic impacts (such as urbanization and road construction) throughout its distribution range, the species has been the subject of several conservation studies. Phytochemical research on the aerial parts of R. mykalea led to the isolation of a new flavonoid, namely patuletin 7-O-(6 ''-E-sinapyl)-beta-glucopyranoside, together with four known flavonoids from n-butanol extract. The structures of all isolated compounds were elucidated by spectroscopic analysis (1D and 2D NMR, HR-ESI-MS), as well as by comparison with the relevant literature data. All of the compounds have been isolated from the genus Rhaponticoides for the first time.