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One of the last remaining floating tongues of the Greenland ice sheet (GrIS), the Petermann Glacier ice shelf (PGIS), is seasonally shielded from warm Atlantic water (AW) by the formation of sea ice arches in the Nares Strait. However, continued decline of the Arctic sea ice extent and thickness suggests that arch formation is likely to become anomalous, necessitating an investigation into the response of PGIS to a year-round mobile and thin sea ice cover. We use a high-resolution unstructured grid 3-D ocean–sea ice–ice shelf setup, featuring an improved sub-ice-shelf bathymetry and a realistic PGIS geometry, to investigate in unprecedented detail the implications of transitions in the Nares Strait sea ice regime, that is, from a thick and landfast sea ice regime to a mobile, and further, a thin and mobile sea ice regime, with regard to PGIS basal melt. In all three sea ice regimes, basal melt near the grounding line (GL) presents a seasonal increase during summer, driven by a higher thermal driving. The stronger melt overturning increases the friction velocity slightly downstream, where enhanced friction-driven turbulent mixing further increases the thermal driving, substantially increasing the local melt. As the sea ice cover becomes mobile and thin, wind and (additionally in winter) convectively upwelled AW from the Nares Strait enter the PGIS cavity. While its effect on basal melting is largely limited to the shallower (<200 m) drafts during winter, in summer it extends to the GL (ca. 600 m) depth. In the absence of an increase in thermal driving, increased melting under the deeper (>200 m) drafts in winter is solely driven by the increased vertical shear of a more energetic boundary layer current. A similar behaviour is noted when transitioning from a mobile to a thin mobile sea ice cover in summer, when increases in thermal driving are negligible and increases in melt are congruent with increases in friction velocity. These results suggest that the projected continuation of the warming of the Arctic Ocean until the end of the 21st century and the accompanying decline in the Arctic sea ice extent and thickness will amplify the basal melt of PGIS, impacting the long-term stability of the Petermann Glacier and its contribution to the future GrIS mass loss and sea level rise. [ABSTRACT FROM AUTHOR]