To reach the goal of large-scale seaweed cultivation in Norway, new knowledge concerning commercially important species like the kelp Saccharina latissima is essential. This includes fundamental understanding of physiological mechanisms like nutrient uptake kinetics to better understand its ecological niche and nutritional requirements. The initial short-term nitrate (NO3−) uptake kinetics in cultivated S. latissima juvenile sporophytes were evaluated under nitrogen-saturated and nitrogen-limited conditions. The uptake was measured for concentrations in a gradient from 2 to 18 μM NO3− which is representative of Norwegian coastal waters in the main growth season of winter and early spring. Preconditioning treatments led to internal nitrogen pools (total tissue nitrogen and intracellular nitrate) that were significantly lower for the N-limited than for the N-saturated sporophytes prior to the experiment. Nitrate uptake rates, related to biomass (V) and intracellular nitrogen content (U), were linearly related to the substrate concentrations for both N-limited and N-saturated sporophytes, indicating that S. latissima requires high ambient nitrate concentrations to maintain rapid growth. The sporophytes with deficient internal nitrogen pools exhibited higher uptake rates of NO3− than sporophytes with higher internal pools of nitrogen. Mathematical modelling was used to investigate the temporal development of total tissue nitrogen (QN) based on the nitrogen-specific uptake rates (U) and revealed a near linear response of U to changes in QN. The model also found that a maximum estimated value for QN was only approached after more than 20 days at external NO3- concentrations of 8 μM. These results expand the physiological understanding of cultivated S. latissima and are important for a sustainable upscaling of seaweed farm production.