The tospoviral RNA-dependent RNA polymerases (RdRp), or L proteins, perform several conserved functions during virus replication in host cells. In this study, an L segment sequence of 9,040 bp from a new tospovirus (family Bunyaviridae) naturally infecting bean (Phaseolus vulgaris L.) plants was characterized. It encodes the largest RdRp gene known yet for this genus, with deduced 2932aa and a molecular mass of approximately 336 kDa. A Lysine-rich C-terminal extension was found, which apart from our isolate, was only recognized in another recently discovered tospovirus infecting Fabaceae, Soybean vein necrosis associated virus (SVNaV). Due to its distinct biological features and L protein-based phylogenetic analysis showing an almost equidistant position in comparison to Eurasian and American Tospovirus groups, as well as the clustering with SVNaV, we suggest the tentative name Bean necrotic mosaic virus for this unique isolate.