The Arctic is experiencing unprecedented moistening, which is expected to have far-reaching impact on global climate and weather patterns. However, it remains unclear whether this newly-sourced moisture originates locally from ice-free ocean regions or is advected from lower latitudes. In this study, we use water vapour isotope measurements in combination with trajectory-based diagnostics and an isotope-enabled AGCM, to assess seasonal shifts in moisture sources and transport pathways in the Arctic. Continuous measurements of near-surface vapour, δ18O, and δD were performed onboard RV Polarstern during the MOSAiC expedition from October 2019 to September 2020. Combining this isotope dataset with meteorological observations reveals that the spatiotemporal evolution of δ18O mimics changes in local temperature and humidity at synoptic to seasonal time scales, while corresponding d-excess changes suggest a seasonal shift in the origin of moisture. Simulation results from the particle dispersion model FLEXPART support these findings, indicating that summer moisture originates from nearby open ocean, while winter moisture comes from more remote sources with longer residence time over sea-ice. Results from a nudged ECHAM6-wiso simulation also indicate that evaporative processes from the ocean surface reproduce summer sotope values, but are insufficient to explain measured winter isotope values. Our study provides the first isotopic characterization of Central Arctic moisture over the course of an entire year, helping to differentiate the influence of local processes versus large-scale vapour transport on Arctic moistening. Future process-based investigations should focus on assessing the non-equilibrium isotopic fractionation during airmass transformation over sea-ice.