This dataset encompasses maximum growth rates derived from photoperiod reaction norm experiments conducted on Arctic and temperate strains of the diatoms Thalassiosira gravida and Thalassiosira rotula, respectively. The study aimed to investigate the growth response of these strains to varying photoperiods, specifically 1h, 4h, 8h, 16h, and 24h light cycles. The experiments were performed using four Arctic strains of T. gravida at the University of Oslo (from 2022-02-15 to 2022-03-02) and four temperate strains of T. rotula at the University of Trondheim (from 2022-07-09 to 2022-07-18). Prior to the experiment, cultures were maintained in climate chambers under controlled conditions, with Arctic strains kept at 4°C and temperate strains at 15°C, both under a light intensity of 30 µmol photons m⁻²s⁻¹ and a photoperiod of 16:8h light:dark. The photoperiod reaction norms assays were conducted in nanocosm well-plate photobioreactors, allowing for high replication and well-specific programmed light settings. The experimental temperature was set to 9°C for Arctic strains and 16°C for temperate strains, representing the lowest optimum temperatures among the studied genotypes. Cultures were acclimated for seven days before the actual growth experiments, with daily measurements of chlorophyll-a fluorescence intensity using a photo-spectrometric plate reader. The growth rates were calculated using the growthrates package after subtracting the mean blank value of the medium from the measured fluorescence values. The dataset provides valuable insights into the photoperiodic growth responses of T. gravida and T. rotula, which can be crucial for understanding the ecological adaptability of these diatom species.