Microalgae have been extensively studied for the production of various products. However, to date, microalgal biomass has not become economically feasible, mainly due to different issues such as contamination from various sources that occurs during downstream processes, and which leads to low quality biomass with limited application. In this study, to overcome contamination by flocculants and other microorganisms, the cationic biopolymer alpha-Poly-L-lysine (alpha-PLL) was applied. The cationic amine moiety and polymeric chain of alpha-PLL rendered microalgal harvesting efficient. With increasing alpha-PLL chain length, efficient dose-and time-dependent harvesting was achieved. In addition to efficient flocculation performance, biomass harvested using alpha-PLL showed suppressed biological contamination through the inherent antimicrobial activity of alpha-PLL. Thus, it is possible to upgrade the quality and storability of produced microalgal biomass using alpha-PLL-induced flocculation.