Magnetic nanoparticles that can be transported in subsurface reservoirs at high salinities and temperatures are expected to have a major impact on enhanced oil recovery, carbon dioxide sequestration, and electromagnetic imaging. Herein we report a rare example of steric stabilization of iron oxide (10) nanoparticles (NPs) grafted with poly(2-acrylamido-2-methylpropanesulfonate-co-acrylic acid) (poly-(AMPS-co-AM) that not only display colloidal stability in standard American Petroleum Institute (API) brine (8% NaCI + 2% CaCl2 by weight) at 90 C for 1 month but also resist undesirable adsorption on silica surfaces (0.4% monolayer NPs). Because the AMPS groups interacted weakly with Ca2+, they were sufficiently well solvated to provide steric stabilization. The PAA groups, in contrast, enabled covalent grafting of the poly(AMPS-co-AA) chains to amine-functionalized 10 NPs via formation of amide bonds and prevented polymer desorption even after a 40000-fold dilution. The aforementioned methodology may be readily adapted to stabilize a variety of other functional inorganic and organic NPs at high salinities and temperatures.