• Nitzschia sigma (Kützing) W.Smith, 1853 (uncertain > unassessed)
• Nitzschia sigma f. brevior Grunow in Van Heurck, 1881 (uncertain > unassessed)
• Nitzschia sigma f. clausii (Hantzsch) Grunow, 1878 accepted as Nitzschia clausii f. clausii Hantzsch, 1860
• Nitzschia sigma f. elongatae Grunow in Van Heurck, 1881 (uncertain > unassessed)
• Nitzschia sigma f. genuina Cleve in Cleve & Jantzsch, 1882 (uncertain > unassessed)
• Nitzschia sigma f. habirshawii (Febiger ex Cleve & Moller) Grunow in Van Heurck, 1881 (uncertain > unassessed)
• Nitzschia sigma f. indica Thomas & Gonzalves, 1965 (uncertain > unassessed)
• Nitzschia sigma f. longissima Grunow in Van Heurck, 1881 (uncertain > unassessed)
• Nitzschia sigma f. longissima Manguin in Bourrelly & Manguin, 1952 (uncertain > unassessed)
• Nitzschia sigma f. major (Hustedt in Schmidt et al.) Cleve-Euler, 1952 (uncertain > unassessed)
• Nitzschia sigma f. maxima (Grunow) Hustedt in Schmidt et al., 1924 accepted as Nitzschia maxima f. maxima Grunow, 1878
• Nitzschia sigma f. maxima Grunow, 1867 (uncertain > unassessed)
• Nitzschia sigma f. maxima Halden, 1929 (uncertain > unassessed)
• Nitzschia sigma f. rigida (Kützing) Grunow, 1878 accepted as Amphipleura rigida Kützing, 1844
• Nitzschia sigma f. sigma (Kützing) W. Smith, 1853 accepted as Synedra sigma Kützing, 1844
• Nitzschia sigma f. valida Grunow, 1878 (uncertain > unassessed)
• Nitzschia sigma subsp. rigida
• Nitzschia sigma var. abludens Grunow, 1878 (uncertain > unassessed)
• Nitzschia sigma var. anguillula (Schumann) Grunow, 1878 accepted as Nitzschia anguillula Schumann, 1867
• Nitzschia sigma var. calamae Frenguelli, 1928 †
• Nitzschia sigma var. consimilis Grunow, 1878 (uncertain > unassessed)
• Nitzschia sigma var. curvula (Ehrenberg) Brun, 1880 accepted as Nitzschia curvula var. curvula W. Smith, 1856
• Nitzschia sigma var. diminuta Grunow in Van Heurck, 1881 (uncertain > unassessed)
• Nitzschia sigma var. elongata Grunow (uncertain > unassessed)
• Nitzschia sigma var. fasciculata Grunow, 1878 (uncertain > unassessed)
• Nitzschia sigma var. fonticola Hustedt, 1937 (uncertain > unassessed)
• Nitzschia sigma var. genuina Grunow, 1878 (uncertain > unassessed)
• Nitzschia sigma var. incisa Mereschkowsky, 1903 (uncertain > unassessed)
• Nitzschia sigma var. indica Karsten, 1907 (uncertain > unassessed)
• Nitzschia sigma var. intercedens Grunow, 1878 (uncertain > unassessed)
• Nitzschia sigma var. latiuscula Grunow in Van Heurck, 1881 (uncertain > unassessed)
• Nitzschia sigma var. major Grunow in Van Heurck, 1881 (uncertain > unassessed)
• Nitzschia sigma var. maxima Grunow, 1878 (uncertain > unassessed)
• Nitzschia sigma var. minor (uncertain > unassessed)
• Nitzschia sigma var. neogena Grunow, 1882 (uncertain > unassessed)
• Nitzschia sigma var. purvula Brun (uncertain > unassessed)
• Nitzschia sigma var. rigida Grunow ex Van Heurck, 1880 (uncertain > unassessed)
• Nitzschia sigma var. rigidula (Peragallo & Peragallo) Grunow, 1881 accepted as Nitzschia rigida var. rigidula Peragallo & Peragallo, 1900
• Nitzschia sigma var. robusta Østrup, 1910 (uncertain > unassessed)
• Nitzschia sigma var. scabra Cleve, 1883 (uncertain > unassessed)
• Nitzschia sigma var. serpentina Skvortzow, 1927 (uncertain > unassessed)
• Nitzschia sigma var. sigmatella Grunow, 1878 (uncertain > unassessed)
• Nitzschia sigma var. subcapitata Rabenhorst, 1864 (uncertain > unassessed)
• Nitzschia sigma var. subrecta (Grunow) Grunow, 1878 (uncertain > unassessed)
• Nitzschia sigma var. undulata Hohn & Hellerman, 1966 (uncertain > unassessed)
• Nitzschia sigmaformis Hustedt
• Nitzschia sigmaformis var. sigmaformis Hustedt, 1955 (uncertain > unassessed)
• Nitzschia sigmaformis var. socialis Karajeva, 1963 (uncertain > unassessed)
• Nitzschia sigmatella Gregory, 1854 accepted as Stenopterobia sigmatella (Gregory) R. Ross in Hartley, 1986 accepted as Nitzschia sigmatella var. sigmatella Gregory, 1855
• Nitzschia sigmatella var. armoricana (Kützing) F.W. Mills, 1934 (uncertain > unassessed)
• Nitzschia sigmatella var. major Grunow, 1862 (uncertain > unassessed)
• Nitzschia sigmatella var. minor Grunow, 1862 (uncertain > unassessed)
• Nitzschia sigmatella var. pusilla Grunow, 1862 (uncertain > unassessed)
• Nitzschia sigmatella var. sigmatella Gregory, 1855 (uncertain > unassessed)
• Nitzschia sigmatella var. subrecta Grunow, 1862 (uncertain > unassessed)
• Nitzschia sigmatoformis (uncertain > unassessed)
• Nitzschia sigmatoformis var. subrecta Proschkina-Lavrenko, 1963 (uncertain > unassessed)
• Nitzschia sigmoidea var. sigmatella (Gregory) (uncertain > unassessed)