Although influenza B viruses (IBVs) can contribute over 50% of seasonal influenza infections and cause severe respiratory disease, investigations of T cell responses to influenza have largely focussed on influenza A viruses (IAVs). To date only a handful of IBV derived T cell epitopes have been identified, primarily through epitope prediction or homology to IAV. Here we used mass spectrometry for unbiased identification of IBV peptides presented by HLA class I and II molecules for functional investigation. We infected human B-lymphoblastoid cell lines with IBV and sequentially immunoaffinity purified HLA-peptide complexes to yield peptides bound to (1) the common Caucasian class I allotype HLA-A*02:01, (2) the remaining class I allotypes HLA-C*04:01 and B*35:03, and (3) the class II HLA molecules (HLA-DRB1*12:01, DRB3*02:02, DQB1*03:01 and DPB1*04:01) of these cells. Eluted peptide ligands were analysed by LC-MS/MS. We identified 71 HLA class I ligands of 8-13 residues length which spanned all proteins in the IBV proteome, except the ion channel BM2 (10/11 proteins). Of these, 58 were assigned as HLA-A*02:01 ligands, commonly displaying hydrophobic HLA-A*02:01 anchor residues at P2 and the C-terminus, and 4 as HLA-C*04:01 ligands, predominantly bearing aromatic or aliphatic residues at P2 and the C-terminus, and Asp at P3. A further 212 HLA class II ligands, principally 12-18 residues in length, were identified from 6/11 IBV proteins. Notably, although dominated by hemagglutinin, HLA class II ligands were also sourced from BM2, suggesting compartmentalisation to the HLA class II loading pathway. Preliminary functional analyses in HLA-A*02:01+ individuals and HLA-A*02:01 transgenic mice determined the immunogenicity of a number of the HLA-A*02:01 IBV derived peptide ligands identified, confirming the potential of this approach to provide candidate peptides for investigation of IBV-specific T cell responses.