This dataset presents the results of temperature performance curve (TPC) experiments conducted on Arctic and temperate diatom species to investigate their thermal adaptation. The study examined five Arctic-adapted strains of Thalassiosira gravida and five temperate-adapted strains of Thalassiosira rotula. The experiments were performed between 2021-12-08 and 2022-04-16 at the Alfred Wegener Institute, Bremerhaven, Germany. The primary objective of this research was to assess and compare the temperature-dependent growth responses of Arctic and temperate diatom strains, providing insights into their thermal plasticity and potential adaptations to different climate regimes. Growth rates were assessed across a temperature gradient specific to each species group. For T. gravida, temperatures ranged from -0.5°C to 15°C, while T. rotula was exposed to temperatures between 2.8°C and 27.8°C. Experiments were conducted using a table thermoblock under controlled light conditions (30 µmol photons m-2 s-1, 16:8 h light:dark cycle). Prior to the experiments, cultures were acclimated to their respective temperature gradients over a period of 15 days. The TPC assays were then initiated with four replicates per strain and temperature, starting with an initial cell density of 250 cells ml-1 in 40 mL of modified K-medium. Daily sampling was performed by fixing 500 µl subsamples with 2% Lugol's solution. Cell densities were quantified using light microscopy until cultures reached the stationary phase. The dataset includes daily cell count data for each strain at each experimental temperature, allowing for the calculation of growth rates and the construction of temperature performance curves. This information can be used to determine optimal growth temperatures, thermal tolerances, and the shape of the thermal reaction norms for these ecologically important diatom species. This study provides valuable data for understanding the thermal ecology of Arctic and temperate diatoms, which is essential for predicting their responses to ocean warming and potential shifts in species distributions under future climate scenarios.