A mannose/glucose‐specific lectin was isolated from seeds of Parkia platycephala, the most primitive subfamily of Leguminosae plants. The molecular mass of the purified lectin determined by mass spectrometry was 47 946 ± 6 Da (by electrospray ionization) and 47 951 ± 9 Da (by matrix‐assisted laser‐desoption ionization). The apparent molecular mass of the lectin in solutions of pH in the range 4.5–8.5 determined by analytical ultracentrifugation equilibrium sedimentation was 94 ± 3 kDa, showing that the protein behaved as a non‐pH‐dependent dimer. The amino‐acid sequence of the Parkia lectin was determined by Edman degradation of overlapping peptides. This is the first report of the primary structure of a Mimosoideae lectin. The protein contained a blocked N‐terminus and a single, nonglycosylated polypeptide chain composed of three tandemly arranged homologous domains. Each of these domains shares sequence similarity with jacalin‐related lectin monomers from Asteraceae, Convolvulaceae, Moraceae, Musaceae, Gramineae, and Fagaceae plant families. Based on this homology, we predict that each Parkia lectin repeat may display a β prism fold similar to that observed in the crystal structure of the lectin from Helianthus tuberosus. The P. platycephala lectin also shows sequence similarity with stress‐ and pathogen‐upregulated defence genes of a number of different plants, suggesting a common ancestry for jacalin‐related lectins and inducible defence proteins.