Berberis cretica L. (Berberidaceae) is a small shrub growing in Anatolia. A previous investigation of its alkaloidal content revealed the presence of protoberberinium salts berberine, palmatine, jatrorrhizine, as well as the aporphine magnoflorine. A reinvestigation of this plant produced a new amorphous phenanthrene alkaloid corydinemethine (1, C₂₁H₂₅NO₄). The structure of corydinemethine was confirmed by Hofmann β-elimination of quaternized (+)-corydine, as the synthetic product was identical to the natural one in NMR, UV, mass spectra, and TLC behavior. Corydinemethine is the first phenanthrene alkaloid known to possess a substituent at the hindered C-5 position and is a C-3, C-4, C-5, C-6 tetraoxygenated phenanthrene alkaloid, which may be biogenetically related to lactonic alkaloids (taspine, chiloenamine, andesine, santiagonamine, aconcaguine) found in Berberidaceae. Additionally, nineteen known alkaloids were detected, including eight bisbenzylisoquinolines ((+)-isotetrandrine, (+)-berbamine, (+)-oxyacanthine, (+)-obaberine, (+)-obamegine, (+)-thalrugosine, (+)-aromoline, (+)-berbamunine), four aporphines ((+)-isoboldine, (+)-thaliporphine, (+)-glaucine, (+)-magnoflorine), and seven protoberberines (berberine, palmatine, jatrorrhizine, columbamine, canadine, berberrubine, oxyberberine).