Abstract Rosmarinic acid and other caffeic and 4‐coumaric acid derivatives are widespread in land plants. These phenolic compounds contribute to the medicinal uses of plants. Ferns (Polypodiopsida), however, are still under‐investigated in terms of phenolic natural products and their biosyntheses. We analysed 32 fern species in the Blechnaceae, encompassing all three subfamilies, for content of caffeoyl‐5‐ O ‐quinic acid, rosmarinic acid and related compounds using LC/ESI‐MS/MS and HPLC. A compound previously unknown from ferns, but known in two species of Lamiaceae, was purified, its structure elucidated by NMR, and named blechnic acid B. While caffeoyl‐5‐ O ‐quinic acid was present in extracts from all investigated species, rosmarinic acid was only found in four species ( Doodia maxima , Lomariocycas tabularis , Neoblechnum brasiliense , Oceaniopteris ciliata) belonging to superclade B of the Blechnoideae subfamily. Blechnic acid B, a novel compound in ferns, is composed of two moieties of isorinic acid (caffeoyl‐4′‐hydroxyphenyllactic acid). This lignan was only detected in Neoblechnum brasiliense , together with three related lignans having one or two 3,4‐dihydroxyphenyllactic acid moieties. Our study suggests that genes for biosynthesis of caffeoyl‐5‐ O ‐quinic acid were already established in ancestor species of the Blechnaceae. Blechnic acid B and its three derivatives might have formed through a selective coupling of the rosmarinic/isorinic acid monomers with the help of a dirigent protein.