Flocculation of marine microalgae is challenging because of the high ionic strength of the culture medium. Most of the polymer-based flocculants undergo coiling and hence perform poorly in the sea water. In this study, we investigate cationic cellulose nanocrystals (CNCs) as a biobased flocculant to flocculate the marine microalgae Nannochloropsis oculata, and compared its performance to chitosan. Cationic CNCs induced flocculation at a dose as low as 11 mg L-1 while chitosan required a dose of 35 mg L-1 in achieving maximum flocculation efficiency of around 90% in seawater medium containing 30 g L-1 synthetic sea salt. The maximum flocculation efficiency achieved by CNCs was about 90%, while chitosan achieved a flocculation efficiency textgreater 95 %. This was attributed to small stable flocs remaining in suspension using CNC flocculants. However, following the flocculation with CNCs, centrifugation for 1 min at 180 x g or gravity filtration using a 30 µm nylon filter resulted in the removal of these small, stable flocs (150 µm diam) resulted in textgreater95% harvesting efficiency of Nannochloropsis oculata. Moreover, the cationic CNCs tested in this study possess a permanent positive charge, allowing flocculation over a wide pH-range (from 4 to 10). As most abundant polymer on earth, cellulose, and its extracted nanocrystals, can serve as a sustainable alternative natural flocculant universally applied in both harvesting fresh water and marine micoralgae.