We present the synthesis and characterization of spherical polyelectrolyte brush (SPB) particles carrying zwitterionic polyelectrolyte chains. The colloidal particles consist of a divinylbenzene cross-linked poly(styrene) core (PS-co-DVB core) of about 100 nm in diameter onto which linear zwitterionic poly(2-(methacryloyloxy)ethyl dimethyl-(3-sulfopropyl)-ammonium hydroxide) (pMEDSAH) chains are chemically grafted via ATRP. zeta-Potential measurements demonstrated that the SPB has an electrophoretic mobility due to the net charge of the PS-co-DVB core particles. There is an increase of the brush thickness L of the zwitterionic brush at high concentrations of sodium chloride at room temperature. Temperature-dependent measurements by dynamic light scattering (DLS) showed that the zwitterionic SPBs swell reversibly with increasing temperature because of the upper critical solution temperature (UCST) of the pMEDSAH chains in water. This effect could be enhanced by the addition of salt. Cryogenic transmission electron microscopy (cryoTEM) showed that the shell of the particles is quite compact at room temperature. However, the hydrodynamic radius as measured by DLS was significantly larger than the particles radius inferred from microscopy. This result is explained in terms of a model in which the shell of the zwitterionic SPB undergoes a phase separation into a dense phase and a few chains sticking out into the aqueous phase.