Flocculation of microalgae is a promising technique to reduce the costs and energy required for harvesting microalgae. Harvesting marine microalgae requires suitable flocculants to induce the flocculation under marine conditions. This study demonstrates that cationic polymeric flocculants can be used to harvest marine microalgae. Different organic flocculants were tested to flocculate Phaeodactylum tricornutum and Neochloris oleoabundans grown under marine conditions. Addition of 10ppm of the commercial available flocculants Zetag 7557 and Synthofloc 5080H to P. tricornutum showed a recovery of, respectively, 98%±2.0 and 94%±2.9 after flocculation followed by 2h sedimentation. Using the same flocculants and dosage for harvesting N. oleoabundans resulted in a recovery of 52%±1.5 and 36%±11.3. This study shows that cationic polymeric flocculants are a viable option to pre-concentrate marine cultivated microalgae via flocculation prior to further dewatering.