Regional sea ice models often do not cover the full extent of polar ice and instead include open ocean boundaries that are not ice-free year-round. This necessitates the specification of lateral boundary conditions for sea ice, an inherently challenging task for most sea ice models. Although this issue is less critical for pan-polar domains, the interior ice state still needs to be constrained for many applications. In this study, we present the design and evaluation of a Newtonian relaxation algorithm for sea ice, implemented in the NOAA Geophysical Fluid Dynamics Laboratory (GFDL) Sea Ice Simulator (SIS2). The algorithm can be applied both at the lateral boundaries to impose open boundary conditions and within the interior domain to constrain sea ice thickness and concentration toward prescribed target fields. The method is flexible and can be applied anywhere in the domain, making it especially well-suited for regional applications of sea ice models with variable ice cover along their boundaries. The method is evaluated within a regional forecasting system based on the NOAA GFDL ocean model (MOM6) coupled with sea ice model (SIS2) for two regional configurations: the Northeast Pacific and the Arctic Ocean. Sensitivity experiments spanning a range of relaxation time scales and nudging strengths demonstrate that the method substantially improves the representation of sea ice and associated ocean surface fields, offering a practical solution for both boundary and interior constraints in regional sea ice modeling.