An in situ technique for preparing composite nanoparticles from hydrophobic cellulose acetate and hydrophilic polysaccharides using nanoprecipitation is presented. This technique allows the nanoparticles’ surface properties to be tuned very specifically. Spherical, narrow-size-distributed composite nanoparticles of different size, charge, functionality, and increased stability can be generated by using hydroxyethyl cellulose, carboxymethyl cellulose, low molecular weight chitosan, and amino cellulose. The influence of the pH and hydrophilic polysaccharide content in the particle formation is shown. The pH- and ionic strength- effective zeta-potential functions are evidence of the presence of functional polysaccharides at the nanoparticle surface. The in situ technique is compared with the adsorption of hydrophilic polysaccharides onto cellulose acetate nanoparticles in two steps. The great potential of in situ prepared composite nanoparticles in the pharmaceutical industry and bio- or food technology, as carriers of hydrophobic substances in aqueous media and for specific surface modifications, e.g., to selectively introduce strong antimicrobial properties, is illustrated.