Understanding the nutrient uptake kinetics of kelp populations will contribute to an improved understanding of environmental adaptation and the breeding of new cultivars. In this study, we examined the morphological characteristics, carbon (C) and nitrogen (N) contents, and NO3 −–N and NH4 +–N uptake kinetics of Undaria pinnatifida sporophytes cultivated at six industrial farms throughout Japan. We detected significant differences in morphology among sites. At Matsushima Bay (northern Pacific coast of Japan), where autumnal seawater temperatures fall in concert with increasing nutrient concentrations, sporophytes were significantly larger than at other sites from December to February. The C content of the sporophytes was seasonally stable at all of the locations, but the N content of sporophytes declined after February due to a decrease in seawater nutrients. We compared the uptake kinetics of NO3 −–N and NH4 +–N among cultivation sites. V max and K s, which are Michaelis–Menten parameters that measure adaptation to nutrient concentrations, were highest in the Seto Inland Sea and lowest in the northern sector of the Sea of Japan. The V max/K s ratio is a measure of adaptation to low nutrient concentrations; the highest values were measured in the northern sector of the Sea of Japan. The parameter ranges were broader than those previously reported for invasive populations of U. pinnatifida in other parts of the world. Thus, we detected population-level adaptations to the various nutrient conditions in Japanese waters, and these results suggest the existence of ecotypes according to nutrient uptake kinetics. The different populations may be used to provide sources of genetic material that could be of value in breeding programmes by improving productivity and quality.