Magnetic doping of semiconductor nanostructures is actively pursued for applications in magnetic memory and spin-based electronics(1,2). Central to these efforts is a drive to control the interaction strength between carriers (electrons and holes) and the embedded magnetic atoms(3-5). In this respect, colloidal nanocrystal heterostructures provide great flexibility through growth-controlled 'engineering' of electron and hole wavefunctions in individual nanocrystals(6,7). Here, we demonstrate a widely tunable magnetic sp-d exchange interaction between electron-hole excitations (excitons) and paramagnetic manganese ions using 'inverted' core-shell nanocrystals composed of Mn(2+)-doped ZnSe cores overcoated with undoped shells of narrower-gap CdSe. Magnetic circular dichroism studies reveal giant Zeeman spin splittings of the band-edge exciton that, surprisingly, are tunable in both magnitude and sign. Effective exciton g-factors are controllably tuned from 200 to C 30 solely by increasing the CdSe shell thickness, demonstrating that strong quantum confinement and wavefunction engineering in heterostructured nanocrystal materials can be used to manipulate carrier-Mn(2+) wavefunction overlap and the sp-d exchange parameters themselves.