The temperature influence on carbon stable isotope discrimination (Δ) in photosynthesis by algae has not been studied taking into account the confounding effect due to photosynthetic rates. This is problematic because usually higher temperatures imply higher photosynthetic rates, and higher photosynthetic rates usually lead to a decrease in Δ. Here, we investigate the effect of temperature on Δ during photosynthesis by Undaria pinnatifida (Harv.) Suringar (Phaeophyta) in a closed system, varying temperatures between 5°C and 20°C and measuring photosynthetic rates simultaneously. There was a general trend of higher Δ for higher temperatures under the same photosynthetic rate, especially for higher photosynthetic rates. These results were consistent with the influence of phenomena related to carbon supply to the plant, like CO2 diffusion in water and through cell membranes. This influence, however, was less strong than that of photosynthetic rates on Δ (lower Δ for higher photosynthetic rates) and can be difficult to observe in nature.