• Abrotanifolia barbata Stackhouse, 1809 accepted as Cystoseira barbata (Stackhouse) C.Agardh, 1820 accepted as Treptacantha barbata (Stackhouse) Orellana & Sansón, 2019
• Abrotanifolia discors (Linnaeus) Stackhouse, 1809 accepted as Cystoseira foeniculacea (Linnaeus) Greville, 1830
• Abrotanifolia foeniculacea (Linnaeus) Stackhouse, 1809 accepted as Cystoseira foeniculacea (Linnaeus) Greville, 1830
• Abrotanifolia loeflingii Stackhouse, 1809 accepted as Cystoseira foeniculacea (Linnaeus) Greville, 1830
• Abrotanifolia mucronata (Turner) Stackhouse, 1809 accepted as Gongolaria nodicaulis (Withering) Molinari & Guiry, 2020
• Acanthochloris bacillifera Pascher, 1938
• Acanthochloris brevispinosa Pascher, 1930
• Acanthochloris brevispinosa var. brevispinosa Péterfi & Péterfi, 1966
• Acanthochloris hungarica Palik (uncertain)
• Acanthochloris ingens Margalef, 1956
• Acanthochloris scherffelii Pascher, 1938
• Acanthochloris verrucosa Ettl, 1977
• Acanthochrysis echinococcus Conrad (uncertain)
• Acanthochrysis echinocystus (Conrad) Conrad (uncertain)
• Acanthochrysis fusiformis Conrad (uncertain)
• Acanthosphaeridium reticulatum M.Hajós & H.Stradner, 1975 †
• Acinetospora asiatica Yaegashi, Yamagishi & Kogame, 2015
• Acinetospora crinita (Carmichael) Sauvageau, 1899
• Acinetospora filamentosa (Noda) Yaegashi, Uwai & Kogame, 2015
• Acinetospora nicholsoniae Hollenberg, 1971
• Acinetospora pusilla (A.W.Griffiths ex Harvey) De Toni, 1895 accepted as Acinetospora crinita (Carmichael) Sauvageau, 1899
• Acinetospora pusilla var. crinita (Carmichael) Batters, 1902 accepted as Acinetospora crinita (Carmichael) Sauvageau, 1899
• Acinetospora vidovichii (Meneghini) Sauvageau, 1898 accepted as Acinetospora crinita (Carmichael) Sauvageau, 1899
• Acrispumella msimbaziensis Boenigk & Grossmann, 2016
• Acrocarpia paniculata (Turner) Areschoug, 1854
• Acrocarpia robusta Womersley, 1964
• Acrocytis groenlandica Rosenvinge
• Acrocytis groenlandicus Rosenvinge, 1933
• Acrospongium ralfsioides Schiffner, 1916
• Acrothrix gracilis Kylin, 1907
• Acrothrix mirabilis Kylin
• Acrothrix neglecta (Batters) Kuckuck, 1929 (uncertain)
• Acrothrix norvegica Levring, 1937 accepted as Acrothrix gracilis Kylin, 1907
• Acrothrix novaeangliae W.R.Taylor
• Acrothrix novae-angliae W.R.Taylor, 1928 accepted as Acrothrix gracilis Kylin, 1907
• Acrothrix pacifica Okamura & Yamada, 1932
• Acrothrix pacifica f. crassa Inagaki, 1954 (uncertain)
• Acrotrichium amphibolis Womersley & Skinner, 1987
• Actinellipsoidion sol (Pascher) Ettl, 1977
• Actinema inaequale Reinsch
• Actinema major Reinsch, 1875
• Actinema scutellum Reinsch, 1875
• Actinema subtilissimum Reinsch, 1875
• Actinocoma ramosa Penard, 1903
• Actinoglena klebsiana O.Zacharias, 1897 accepted as Synura klebsiana (O.Zacharias) Lemmermann, 1899
• Actinomonas mirabilis Kent, 1880
• Actinomonas pusilla Kent (uncertain)
• Actinomonas radiosa G.Roskin (uncertain)
• Actinomonas vernalis A.Stokes, 1885
• Actinophrys pontica Valkanov, 1940
• Actinophrys salsuginosa D.J.Patterson, 2001
• Actinophrys sol (O.F.Müller) Ehrenberg (uncertain)
• Actinophrys tauryanini Mikrjukov & D.J.Patterson, 2001 (uncertain)
• Acystis heinii Schiffner, 1934 accepted as Polycladia heinii (Schiffner) Draisma, Ballesteros, F.Rousseau & T.Thibaut, 2010
• Adenocystis californica Ruprecht, 1850 accepted as Coilodesme californica (Ruprecht) Kjellman, 1889
• Adenocystis durvillei (Bory) Hooker f. & Harvey, 1847 accepted as Colpomenia durvillei (Bory de Saint-Vincent) M.E.Ramírez, 1991
• Adenocystis lessonii (Bory) Hooker f. & Harvey, 1845 accepted as Adenocystis utricularis (Bory) Skottsberg, 1907
• Adenocystis longissima (Skottsberg) Asensi, Delépine, Rousseau & Reviers, 2004
• Adenocystis rimosa (Montagne) Asensi, Delépine, Reviers & Rousseau, 2003
• Adenocystis utricularis (Bory) Skottsberg, 1907
• Adriogloia adriatica Ercegovic, 1955
• Aegira virescens (Carmichael ex Berkeley) Setchell & N.L.Gardner, 1924 accepted as Eudesme virescens (Carmichael ex Berkeley) J.Agardh, 1882
• Aegira zosterae Fries, 1835 accepted as Eudesme virescens (Carmichael ex Berkeley) J.Agardh, 1882
• Aeronema polymorphum J.W.Snow, 1911 accepted as Heterococcus polymorphus (J.W.Snow) Vischer, 1936
• Agarum asiaticum De la Pylaie, 1830
• Agarum brassiciforme (Postels & Ruprecht) Setchell & Gardner
• Agarum brodiei (Turner) Trevisan, 1845 accepted as Coccotylus truncatus (Pallas) M.J.Wynne & J.N.Heine, 1992
• Agarum clathratum Dumortier, 1822
• Agarum clathratum subsp. yakishiriense Yamada ex G.H.Boo & P.C.Silva, 2011
• Agarum clathrus (S.G.Gmelin) Greville, 1830 accepted as Thalassiophyllum clathrus (S.G.Gmelin) Postels & Ruprecht, 1840
• Agarum corallinum Nees, 1820 accepted as Griffithsia corallinoides (Linnaeus) Trevisan, 1845
• Agarum costatum (C.Agardh) Dumortier, 1822 accepted as Costaria costata (C.Agardh) De A.Saunders, 1895
• Agarum costatum (Turner) Gaillon, 1828 accepted as Costaria costata (C.Agardh) De A.Saunders, 1895
• Agarum crenulatum (J.Agardh) Trevisan
• Agarum cribrosum Bory, 1826 accepted as Agarum clathratum Dumortier, 1822
• Agarum cribrosum f. rishiriense I.Yamada, 1974 (uncertain)
• Agarum cribrosum f. rugosum I.Yamada, 1974 (uncertain)
• Agarum cribrosum f. yakishiriense (Y.Yamada) I.Yamada, 1974 accepted as Agarum clathratum subsp. yakishiriense Yamada ex G.H.Boo & P.C.Silva, 2011
• Agarum cribrum Gaillon, 1828 accepted as Agarum clathratum Dumortier, 1822
• Agarum delisei Bory de Saint-Vincent, 1826 accepted as Alaria esculenta (Linnaeus) Greville, 1830
• Agarum esculentum (Linnaeus) Bory de Saint-Vincent, 1826 accepted as Alaria esculenta (Linnaeus) Greville, 1830
• Agarum fimbriatum Harvey, 1862 accepted as Neoagarum fimbriatum (Harvey) H.Kawai & T.Hanyuda, 2017
• Agarum gmelinii Mertens ex Postels & Ruprecht, 1840 accepted as Agarum clathratum Dumortier, 1822
• Agarum gmelinii var. undulatum Postels & Ruprecht (uncertain)
• Agarum herediae (Clemente) Trevisan, 1845 accepted as Phyllophora herediae (Clemente) J.Agardh, 1842
• Agarum membranifolium (Goodenough & Woodward) Trevisan, 1845 accepted as Phyllophora pseudoceranoïdes (S.G.Gmelin) Newroth & A.R.A.Taylor ex P.S.Dixon & L.M.Irvine, 1977
• Agarum membranifolium var. fibrillosum (C.Agardh) Trevisan (uncertain)
• Agarum membranifolium var. fimbriatum (Hudson) Trevisan (uncertain)
• Agarum membranifolium var. latifolium (Turner) Trevisan (uncertain)
• Agarum membranifolium var. pinnatum (C.Agardh) Trevisan (uncertain)
• Agarum nervosum (De Candolle) Trevisan (uncertain)
• Agarum nervosum var. spirale (C.Agardh) Trevisan (uncertain)
• Agarum nervosum var. undulatum (C.Agardh) Trevisan (uncertain)
• Agarum oharaense Yamada, 1961 accepted as Neoagarum oharaense (Yamada) H.Kawai, 2017
• Agarum pertusum (K.H.Mertens) Postels & Ruprecht, 1840
• Agarum pertusum f. brassicaeformis Postel & Ruprecht, 1840
• Agarum pertusum var. platyneurum Postel & Ruprecht, 1840
• Agarum proliferum (Lightf.) Trevisan (uncertain)
• Agarum proliferum (Roth) Nees (uncertain)
• Agarum pylaiei Bory, 1826 accepted as Alaria pylaiei (Bory) Greville, 1830