Harvesting is one of the major bottlenecks to the large-scale expansion of microalgae for production of food, feed, bulk chemicals, or biofuels. The objective of this study was to evaluate the efficiency of Tanfloc, a low molecular weight tannin-based cationic polymer, for harvesting the marine microalga Nannochloropsis oculata, a promising feedstock for biofuels. A 2^4 full factorial bench scale (0.25 L) experiment determined the effects of salinity (10, 30), pH (6, 8), polymer dosage (1, 10 mg/L), and biomass concentration (200, 400 mg/L) on flocculation. Afterwards, the best harvesting conditions were replicated at pilot scale (250 L) to verify up-scaling potential and reproducibility over time. Overall, increasing Tanfloc dosage improved efficiency (21.5%) whereas pH (-20.4%) and salinity (-41.7%) hampered flocculation. Bench scale conditions resulting in higher flocculation efficiency (99%) were 10 mg/L of Tanfloc, pH 6, and salinity 10. No difference was observed (p < 0.05) between bench and pilot scale results, leading to conclude that Tanfloc is a reliable polymer for harvesting N. oculata cultured in brackish water. Moreover, it was observed that Tanfloc outperformed synthetic polymers recently reported for harvesting marine microalgae.