There are no reports of star polymers synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization in aqueous dispersions (e.g. emulsion, miniemulsions). In this work, we used two RAFT agents to produce linear and 4-arm star polymers in a well-controlled miniemulsion system using conventional surfactants (sodium dodecyl sulfate and hexadecane) and initiators (ammonium persulfate). The miniemulsions were formed through ultrasonication of the polymerization mixture to produce very small droplet sizes. Polymerization of styrene in these miniemulsions resulted in the formation of polymer nanoparticles of approximately 60 and 75 nm with narrow particle size distributions regardless of the targeted molecular weight. The rate of polymerization in these miniemulsions (100% conversion after 3.3 h) was significantly faster than in solution (60% conversion after 20 h). The reason for this fast rate was due to compartmentalization of the radicals within the growing particles. In addition, narrow MWDs were produced with polydispersity (PDIMWD) values below 1.22 for the linear and below 1.09 for the 4-arm star polymers. The particle size distributions were found to be narrow, less than 0.055 for the linear and less than 0.113 for the 4-arm star polymers. This is the first successful miniemulsion, to our knowledge, of 4-arm stars by RAFT to create both narrow molecular weight and narrow particle size distributions. The properties of such polymer latex particles are yet to be explored but they should have vastly different properties compared to latex particles with linear polymers. Our work opens a synthetic route to this class of latex particles previously unobtainable using conventional reagents, and providing exciting opportunities for various applications.