Magnetic nanoparticles with high mobility 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 present a rare example of elelctrosteric stabilization of iron oxide (IO) nanoparticles (NPs) grafted with poly(2-acrylamido-2-methylpropanesulfonate-co-acrylic acid) (poly(AMPS-co-AA)) that not only display colloidal stability in standard American Petroleum Institute (API) brine (8% NaCl + 2% CaCl2 by weight) at 90 °C for 1 month but also resist undesirable adsorption on silica surfaces (0.4% monolayer NPs). In this copolymer coating, the AMPS groups interact weakly with Ca2+, so that the poly(AMPS-co-AA) chains grafted onto IO NPs surface remain sufficiently well solvated and negatively charged to provide electrosteric stabilization. The AA groups, in addition, enable covalent grafting of the poly(AMPS-co-AA) chains to amine-functionalized IO NPs via formation of multiple amide bonds and prevent polymer desorption even after a 40 000-fold dilution. The remarkable stability and transport properties of poly(AMPS-co-AA) grafted IO NPs is attributed to the high stability of poly(AMPS-co-AA) chains as revealed by dynamic light scattering and electrokinetic measurements. The low equilibrium silica adsorption capacity of poly(AMPS-co-AA) grafted IO NPs correlates well with the low retention in 1D column flow experiments. The aforementioned methodology may be readily adapted to stabilize a variety of other functional inorganic and organic NPs at high salinities and temperatures.