The unique properties of magnetic nanocrystals have triggered intensive research towards their effective functionalization and application in many technological fields. Although synthesis of magnetic. colloids is being thoroughly studied, there is limited knowledge on the synthesis, characterization, and properties of magnetic polyelectrolyte spherical brushes. In the present work, the preparation of such hybrids and the subsequent formation of stable aqueous colloids are described. The core of the spherical brush consists of a magnetic gamma-Fe2O3 nanocrystallite (faceted but mostly spherical-like) with a mean diameter of 17 nm. The bioadhesive polyelectrolyte poly(sodium 4-styrene sulfonate), forming the surrounding brush layer, was proven to be an effective covalently modifying macromolecule for the iron oxide surface, as Fourier transform IR spectroscopy revealed. Several observations on colloidal aspects are discussed and are successfully explained by models and experiments describing polyelectrolyte brushes with a soft polymeric core. Finally, the hybrids exhibit their multifunctional character and their technological importance by combining in a single and soluble product with magnetic and nonlinear optical properties.