Water treatment with aluminum and iron salts has caused environmental problems. Moringa oleifera seed flour contains coagulant proteins which have been used in drinking water treatment. The use of plant extracts increase organic material in water. In this study we described the isolation and characterization of a saline soluble M. oleifera seed lectin (SSMoL) and investigation of coagulant properties. SSMoL was isolated after seed flour extraction in 0.15 M NaCl followed by guar gel column chromatography and 1.0 M salt elution. The lectin was evaluated for hemagglutinating activity (HA) with human and rabbit erythrocytes, pH values, temperature as well as in the presence of ions, carbohydrates and glycoproteins. The coagulant activity was measured with water sample treated with kaolin clay; the reduction in absorbance (500 nm, OD₅₀₀) in relation to control defined coagulation activity. SDS-PAGE and PAGE for basic and acidic native proteins were performed. SSMoL showed HA to rabbit (titer: 256^–1) and A, B, O and AB (titer: 64^-1, 128^–1, 16^-1 and 256^-1, respectively) human fresh erythrocytes; lectin was active at pH range 4.0 to 9.0 and in the presence of Mg²⁺, Ca²⁺ and  K⁺. SSMoL was resistant up to seven hours at 100° C. Inhibition assays revealed that HA was partially abolished with glucose, lactose, arabinose, trehalose, rhamnose, fetuin, azialofetuin, ovalbumin, thyroglobulin and casein; lectin was completely inhibited with azocasein. SSMoL showed coagulation activity. PAGE to native proteins confirmed SSMoL as a basic protein, not stained with Schiff reagent. The native molecular weight by Sephacryl S-300 gel filtration chromatography was 30.000 Daltons. In conclusion, M. oleifera seeds contain a lectin with coagulant properties which may be used to water purification.