Antimony (Sb) pollution has been of growing environmental concern. Little information is available on biosorption of Sb. In the present study, biosorption behavior and mechanisms of Sb(III) to the cyanobacterium Synechocystis sp. cells were investigated by batch experiments and FTIR analysis. Our study shows that Synechocystis sp. cells are a good adsorbent for Sb(III) with a sorption capacity of 4.68 mg·g^-1 dry weight adsorbent. The isothermal sorption data can be described by the Langmuir Isotherm and the Freundlich Isotherm. Several mechanisms were involved with biosorption of Sb to Synechocystis sp. cells and sorption to binding sites might be dominant. The sorption kinetics followed the pseudo-second order model. The adsorbed Sb is mainly located in extracellular polysaccharides (EPS) and within the cell, and a small proportion was adsorbed onto the cell wall. The proteins and polysaccharides in EPS and the polysaccharides on the cell are the main functional groups that are responsible for adsorption of Sb to Synechocystis sp. cells.