Transgenerational effects (effects of parental environment on offspring traits) have recently gained attention as a means of fast response to changing environmental conditions, e.g. under climate change. In temperate and polar rocky coastal ecosystems, kelps form the base of complexly structured and highly diverse species associations. This study investigates the potential for temperature-mediated transgenerational plasticity along the development from haploid parents (gametophytes) to their juvenile diploid offspring (sporophytes) in a geographically isolated population of the brown alga Laminaria digitata from the island of Helgoland (North Sea). We sampled spores from wild donor sporophytes, which we raised at 5 and 15 °C during gametogenesis, and reared the resulting young sporophytes for three months in a full-factorial split design while keeping genetic lineages separate. A concluding 12-day experiment on growth, biochemistry (carbon, mannitol content) and photosynthetic characteristics (maximum quantum yield Fv/Fm, maximum electron transport rate rETRmax) of five genetic lineages allowed for the separation of late temperature (12 day) responses, within-generation plasticity (early temperature) and transgenerational plasticity (gametogenesis temperature) in response to 5 and 15 °C. We observed significant two- and three-way interactions between gametogenesis temperature and early and late sporophyte temperatures for all parameters. While interactive effects between early and late experimental temperatures probably represent acclimation processes, interactions involving gametogenesis temperature indicate transgenerational effects. The direction of these effects differed between parameters. A main effect is that only with a history of 5 °C as gametogenesis and early temperature, sporophytes are growing faster at 5 than 15 °C over 12 days late temperature. According to our results, the temperature experienced by parents during gametogenesis influences temperature reaction norms in three- to four-month-old Laminaria digitata sporophytes. This, to our knowledge, is the first evidence for transgenerational plasticity in kelps.