We have described the development of a simple, versatile, magnetic, aqueous nanoparticle dispersion stabilized by an ultrathin, strongly surface-bound, polymeric, steric stabilization layer. These coated particles suffer no measurable polymer desorption at high dilutions, but they can also be concentrated sufficiently to form a spiking ferrofluid (>1017 particles/mL) that remains stable under challenging solution conditions for at least several
months.