The negative effect of multivalent cations such as Ca2+ and Mg2+ on the flocculation efficiency of anionic polymeric flocculants has proven to be a difficult challenge to overcome in recent years. We hereby introduce a new series of ultrahigh molecular weight four-arm star-shaped AB block copolymers, with terminal anionic blocks and a neutral core comprising polyacrylic acid and polyacrylamide, respectively. These polymers were successfully synthesized using a two-stage aqueous reversible addition-fragmentation chain transfer (RAFT) polymerization process with the aid of a four-arm star RAFT agent and a redox initiation pair of ammonium persulfate and sodium formaldehyde sulfoxylate. Once synthesized, these polymers were directly used for flocculation analysis where they were compared to a control statistical copolymer of similar molecular weight. The control copolymer was designed to be representative of current commercial flocculants where the anionic functionalities were distributed randomly along the polymer chain, where the negative effects of multivalent cations are heightened. In comparison to the control copolymer, four-arm star-shaped AB block copolymers exhibited strong stability and flocculation efficiency across the three different Ca2+ concentrations (0.05, 0.10, and 0.50 M) employed. Fast settlement rates and low supernatant turbidities were obtained when four-arm star-shaped AB block copolymers were employed, while the flocculation efficiency of the control copolymer was impacted significantly with the increase in the concentration of Ca2+ . With further optimization and analysis, these polymers have a strong potential to be implemented in industrial applications of flocculation in the near future.