The third part of a revision of linear-leaved species of the genus Potamogeton in the Czech Republic is given. Two species, P. obtusifolius and P. friesii, are dealt with in this contribution. Species descriptions, relevant synonyms, illustrations, a list of specimens examined and distribution maps are provided. P. obtusifolius grows almost exclusively in the Bohemian part, particularly in S and E Bohemia, whereas in Moravia it is confined to a few localities mainly in its NE part (Silesia). It is classified as a threatened species of the Czech flora. P. friesii has been collected only in a few localities in C and NE Bohemia. Last time it was seen in 1989 but has not been confirmed in the site since then; thus the species belongs among missing (and probably extinct) taxa in the Czech Republic.