Magnesiosadanagaite, ideally NaCa2[Mg3(A1, Fe3+)2]Si5A13O22(OH)2, is a new member of the amphibole group occurring as rims of zoned amphibole in rock samples from the contact aureole in Kasuga-mura, central Japan. The amphibole is associated with phlogopite, titanite, calcite, pyrrhotite and chalcopyrite, and composed of an A1-poorer core and A1-richer rim. The core and rim are classified into pargasite [A1 = 1.84–2.19 atoms per formula unit (apfu); for O = 23] and pargasite-magnesiosadanagaite (A1 = 2.45–2.84 apfu), respectively. Both have relatively high [A]Na (0.67–0.96 apfu) and low K content (< 0.29 apfu). The high [A]Na (0.70–0.92 apfu) and low K (0.09–0.28 apfu) values of magnesiosadanagaite refute the view that high K substitution at the A site is an essential feature of Si-poor calcic amphiboles. The crystal structure of magnesiosadanagaite was refined for two compositionally different samples. Magnesiosadanagaite has the lowest O(5)-O(6)-O(5) angles (160.4° and 161.8°) ever reported in C2m amphiboles, indicating maximal kinking of the double-chain of tetrahedra.