An increasing number of biomedical applications requires detailed knowledge of the magnetic susceptibility of individual particles. With conventional techniques it is very difficult to analyze individual particles smaller than 1 µm. The authors demonstrate how the susceptibility of individual nanoparticles can be determined in an efficient way by optically analyzing the confined Brownian motion of a nanoparticle trapped in a known magnetic potential well on a chip. A setup is introduced that has a controllable two-dimensional magnetic potential well, which is defined by an integrated microscopic current wire. Susceptibility measurements have been performed on 150–450 nm superparamagnetic beads. They found differences in bead susceptibility of an order of magnitude and differences in volumetric susceptibility of more than a factor of 2.