We introduce the concept of a spintronic effective mass for spin-polarized carriers in semiconductor structures, which arises from the strong spin-polarization dependence of the renormalized effective mass in an interacting spin-polarized electron system. The majority-spin many-body effective mass renormalization differs by more than a factor of 2 at r(s) = 5 between the unpolarized and the fully polarized two-dimensional system, whereas the polarization dependence (approximately 15%) is more modest in three dimensions around metallic densities (r(s) approximately 5). The spin-polarization dependence of the carrier effective mass is of significance in various spintronic applications.