- Macrocystis C.Agardh, 1820
- Marginariella Tandy, 1936
- Masonophycaceae O.C. Schmidt, 1937
- Masonophycus Setchell & N.L.Gardner, 1930
- Melanosiphon M.J.Wynne, 1969
- Melastictis Reinsch, 1890
- Mesogloia C.Agardh, 1817
- Mesogloiopsis Womersley & Bailey, 1987
- Mesosporaceae J. Tanaka & Chihara, 1982
- Microcoryne Strömfelt, 1888
- Microspongium Reinke, 1888
- Microsporangium
- Microzonia J.Agardh, 1894
- Mikrosyphar Kuckuck, 1895
- Mutimo H.Kawai & T.Kitayama, 2012
- Myagropsis Kützing, 1843
- Myelophycus Kjellman, 1893
- Myriactis Kützing, 1843
- Myriactula Kuntze, 1898
- Myriocladia J.Agardh, 1841
- Myriodesma Decaisne, 1841
- Myriogloea P.Kuckuck ex F.Oltmanns, 1922
- Myrionema Greville, 1827
- Myrionemataceae Nägeli, 1847
- Myrionemopsis P.Dangeard, 1968
- Myriotrichia Harvey, 1834
- Myriotrichiaceae