Multiple isoforms of starch synthases (SSs) have been found in plants. In addition, at least two isoforms of granule-bound SS (GBSS) and SSII have further diverged in cereals into two or three subisoforms. Here, we report the occurrence, phylogeny, and expression patterns of two different forms of both GBSSI and SSII in four legumes: birdsfoot trefoil, cowpea, mung bean, and soybean. The phylogenetic data acquired indicate that the putative proteins of both SS duplicates have diverged into two different isoforms: GBSSIa and GBSSIb, and SSIIa and SSIIb. The SSIIb genes appear to have become non-functional in soybean as a result of two nonsense mutations in the putative coding region. Transcripts of the GBSSIa and SSIIa genes were found to be abundant in cotyledons, but had lower expression levels in the leaves of the two starchy seed legumes. However, these genes were expressed at moderate levels in the leaves of the two oilseed legumes. In contrast, the GBSSIb and SSIIb genes were mainly expressed in the leaves of the legumes we examined. In both the legume and cereal species we studied, the GBSS orthologs that were mainly expressed in sink tissues, were more hydrophilic and may have been subjected to more intense purifying selection than those that were mainly expressed in source tissues. These findings provide evidence that the GBSSI and SSII genes in the starchy seed legumes and cereals studied have undergone convergent evolution with respect to evolutionary constraints, amino acid sequences, and expression divergence after gene duplication.