Co-circulation of two influenza B virus lineages, B/Yamagata and B/Victoria, has been recognized since the late 1980s. The assessment of the prevalent lineage and the group of viruses in circulation is of importance in order to decide on the vaccine composition and evaluate its efficacy. The molecular characterization of influenza B viruses in circulation has been the aim of this study; this was approached by identifying and locating nucleotide substitutions in the influenza B virus hemagglutinin (HA) and neuraminidase (NA), specific for the lineage and/or clade. By the alignment of 3456 sequences from the influenza GISAID EpiFlu database, a high number of lineage- and group-specific nucleotide positions have been observed in the HA gene, but not in the NA gene. Additionally, an RT-PCR method has been developed, applicable directly to clinical specimens, which amplifies a short HA region that includes a group of unique molecular signatures. Twenty eight influenza B virus-positive respiratory specimens, collected in Tuscany in the seasons 2012-2013 and 2013-2014, were analyzed. The results revealed two clearly distinguishable patterns: one, more frequent, was characterized by all of the nucleotide changes associated with the B/Yamagata lineage (in most cases of Group 2), whereas the other exhibited all of the changes associated with the B/Victoria lineage. It can be concluded that the analysis of this short HA sequence can permit a rapid, highly sensitive determination of influenza B virus lineages and clades.