Extracellular organic matter (EOM), which is pervasive in harmful algal bloom water, adversely affects human health and the treatment of such water. In this study, we used Tanfloc, a natural flocculant, in EOM treatment to study its flocculation performance on dissolved extracellular organic matter (dEOM) and bound extracellular organic matter (bEOM), as well as its flocculation mechanism at various pH levels. The removal performance was stronger on bEOM than on dEOM, indicated by the higher dissolved organic carbon (DOC) removal rate and the lower Tanfloc dosage in bEOM. The high molecular weight (MW) organic components ( > 100 kDa) and protein-like substances in bEOM were removed adequately, and the humic acid, fulvic acid, and soluble microbialproduct-like substances, mainly in dEOM, were difficult to remove. Tanfloc could remove bEOM and dEOM satisfactorily, mainly by combining with protein and polysaccharide. Removability depended on water pH and occurred in the order pH 4.5 > pH 8.0 > pH 10.5. Tanfloc could flocculate EOM effectively, even at pH 10.5. Charge neutralization and electrostatic patching and bridging were the main mechanisms at pH 4.5, 8.0, and 10.5, respectively. The hydrogen bonds and hydrophobic interaction were conducive to removing specific EOM. The C-O/C-N functional groups in bEOM had stronger interaction with Tanfloc compared with dEOM; consequently, the removal of bEOM was superior. Our results provide guidance to improve the flocculation efficiency on EOM in harmful algal blooms in the effort to reduce the attendant environmental risks.