Sargassum cymosum (Phaeophyta) is a very frequently algae at the Brazil coast. Lectins are proteins or glycoproteins which through their binding site, interact with carbohydrates. The aim of this work was to identify and isolate S. cymosum algae lectin (ScyAL). The extract (10%, w/v) was prepared in 0.15 M NaCl by 16 h at 4 ºC. The extract was treated with ammonium sulphate in different concentrations (0-20%, 20-40%, 40-60% and 60-80%). Hemagglutinating activity (HA) from fractions was evaluated with human (types A, B, O and AB) and rabbit erythrocytes. The 0-20% fraction (F0-20%) was used to isolate the lectin since it showed the highest specific HA (SHA, 40) and protein concentration (6.4 mg). Inhibition assay of HA from F0-20% was performed with carbohydrates (arabinose, celobiose, fructose, galactose, glucuronic acid, manose, metil-α-manopiranoside, N-acetyl-glucosamine, raffinose, rhamnose, sucrose, threalose and xylose) and glycoproteins (asialofetuin, azocasein, casein, ovalbumin and thyroglobulin). ScyAL (0.4 mg), eluted from chitin column with 1.0 M acetic acid, was purified 8 times and showed SHA of 320 (rabbit erythrocytes). ScyAL recognized all human erythrocyte types. The HA inhibitory assay revealed that, of all carbohydrates tested, only glucuronic acid was not inhibitor and the HA was totally inhibited by casein. In conclusion, the established protocol contained two steps yielded ScyAL with high activity.