The influenza A virus RNA polymerase is a heterotrimer, and transcribes and replicates the viral genome in the cell nucleus. Newly synthesised RNA polymerase subunits must therefore be imported into the nucleus during an infection. While various models have been proposed for this process, the consensus is that the PB1 and PA subunits form a dimer in the cytoplasm and are transported into the nucleus by the β-importin RanBP5, with the PB2 subunit imported separately to complete the trimeric complex. In this study, we characterised the interaction of PB1 with RanBP5 further and assessed its importance for viral growth. In particular, we found that the N-terminal region of PB1 mediates its binding to RanBP5 and that basic residues in a nuclear localisation signal are required for RanBP5 binding. Mutating these basic residues to alanines does not prevent PB1 forming a dimer with PA, but does reduce RanBP5 binding. RanBP5-binding mutations reduce, though do not entirely prevent, the nuclear accumulation of PB1. Furthermore, mutations affecting RanBP5 binding are incompatible with or severely attenuate viral growth, providing further support for a key role for RanBP5 in the influenza A virus life cycle.