Understanding the physiological responses of macroalgae to changing temperatures is crucial to predict future scenarios ecosystems will face due to climate change. The two phenotypically similar kelp species, Laminaria digitata and Saccharina nigripes, inhabit overlapping ecological niches in the Arctic but their southern distribution boundary is dissimilar. This suggests a different adaptation towards temperature as their life cycle stages have to withstand a broad temperature range from Arctic to Sub-Arctic or temperate waters. The present study, comparatively investigated the respective two life cycle stages (sporophytes and gametophytes) of both kelp species, starting from unialgal clonal cultures. Gametophytes and sporophytes were subjected to temperature gradients in the laboratory in a full-factorial approach. Gametogenesis, sporophyte recruitment and gametophyte survival was followed in gametophytes. Growth rate, photosynthetic quantum yield and chlorophyll fluorescence vs. irradiance curves (PI-curves) in sporophytes all over two weeks. L. digitata gametophytes and sporophytes survived 24 and 22 °C and thereby 4 and 5 °C higher temperatures than S. nigripes gametophytes and sporophytes, respectively. Although both species recruited between 0-10 °C and L. digitata also at 15 °C. S. nigripes sporophytes showed a much better performances at 0 and 5 °C than L. digitata in growth, but not in photosynthetic parameters. The overall temperature performance of S. nigripes that was investigated here for the first time shows a true Arctic affinity, while L. digitata behaves similarly than populations from temperate populations. Future increase in Arctic seawater temperatures, especially over winter, most probably will reduce presence of S. nigripes. Whether both species take the same or different functional roles in Arctic kelp forest communities is however still non-resolved.