- Acinetospora Bornet, 1891
- Acinetosporaceae Hamel ex J. Feldmann, 1937
- Acrocarpia Areschoug, 1854
- Acrocytis Rosenvinge, 1933
- Acrospongium Schiffner, 1916
- Acrothrix Kylin, 1907
- Acrotrichium Womersley & Skinner, 1987
- Actinema Reinsch, 1874-1875
- Acystis Schiffner, 1934
- Adenocystaceae F. Rousseau, B. de Reviers, M.-C. Leclerc, A. Asensi & R. Delépine
- Adenocystis J.D.Hooker & Harvey, 1845
- Adriogloia Ercegovic, 1955
- Agarum Dumortier, 1822
- Akkesiphycaceae H. Kawai & H. Sasaki
- Akkesiphycus Yamada & Tak.Tanaka, 1944
- Alaria Greville, 1830
- Alariaceae Setchell & Gardner, 1925
- Analipus Kjellman, 1889
- Antarctosaccion Delépine, 1970
- Anthophycus Kützing, 1849
- Apistonema Pascher, 1925
- Arthrocladia Duby, 1830
- Arthrocladiaceae Chauvin, 1842
- Arthrothamnus Ruprecht, 1848
- Ascocyclus Magnus, 1874
- Ascophyllum Stackhouse, 1809
- Ascoseira Skottsberg, 1907
- Ascoseiraceae Skottsberg, 1907
- Ascoseirales
- Ascoseirophila A.F.Peters, 2003
- Asperococcaceae
- Asperococcus J.V.Lamouroux, 1813
- Asterocladaceae
- Asterocladales
- Asterocladon D.G.Müller, E.R.Parodi & A.F.Peters, 1999
- Asteronema Delépine & Asensi, 1975
- Asteronemataceae
- Asterotrichia Zanardini, 1843
- Aureophycus H.Kawai, T.Hanyuda, Lindeberg & S.C.Lindstrom, 2008
- Australofilum A.F.Peters, 2003
- Austronereia Womersley, 1987
- Axillariella P.C.Silva, 1959