The decadal variability and long-term trend of upper-ocean temperature in the Southeast Indian Ocean since the 1950s are investigated based on an eddy-resolving ocean model and two observation-based products. All three datasets show increasing trends and significant decadal changes in both Southeast Indian Subantarctic Mode Water (SEISAMW) core layer temperature and mixed layer temperature (MLT). The temperature change in the SEISAMW core layer is induced by subducted heat change that is in turn due to a change in the MLT on both long-term trend and decadal time scale. A heat budget analysis finds that the long-term increase of MLT is mainly resulted from a combination of more heat flux from the atmosphere to the ocean and shallower mixed layer depth, while both the intensified zonal wind stress and the vertical entrainment have a negative contribution to the increase in the MLT. The decadal variability of MLT is dominated by changes of surface thermal forcing and horizontal advection, with the former contributing more than the latter. The change in the surface thermal forcing is a result of changes of both surface heat flux and mixed layer depth, and the change in the horizontal advection is due mainly to the change in the zonal wind stress. Further analysis suggests that the decadal changes of surface heat flux and zonal wind stress in the SEISAMW formation region are dominated by local zonal and meridional sea-level pressure gradients, respectively.