Seaweed is gaining interest as a sustainable source of protein. An often-used method for protein extraction from seaweed is alkaline extraction. In this study, the robustness of alkaline protein extraction was tested against protein extraction at neutral pH for sugar kelp (S. latissima) harvested at different time points and at different cultivation sites. The harvest time and place significantly influenced the total nitrogen (N) content of the biomass, the N extraction yield as % of biomass N, and the protein solubility. However, there was no specific pattern regarding harvest season and biomass age. The N extraction yield as % of biomass dry matter content was more similar between biomasses. The N extraction yield (% of biomass N) varied from 1.59 % to 25.22 % (mean: 6.61 %) for the control extraction, whereas it was 5.32–52.96 % (mean: 16.03 %) for the alkaline extraction. Hence, the alkaline extraction increased the N yield, and gave a higher N concentration in the protein extracts and improved the solubility of the protein extracts across the different biomass batches compared to the control method at neutral pH. This states the robustness of the alkaline protein extraction for sugar kelp and suggests improved suitability in foods compared to the extraction at neutral pH.