The zwitterionic phospholipid polymer poly(2-methacryloyloxyethyl phosphorylcholine-co-n-butyl methacrylate) (PMB) is amphiphilic copolymer, and it has been reported to directly penetrate cell membranes and have good cytocompatibility. Conventional PMBs are linear-type random copolymers that are polymerized by a free radical polymerization technique. In contrast, star-shaped polymers, or simple branched-type polymers, have unique properties compared to the linear types, for example, a viscosity based on the effect of the excluded volume. In this study, a branched architecture was introduced into a PMB molecular structure, and a 4-armed star-shaped PMB (4armPMB) was synthesized by an atom transfer radical polymerization (ATRP) technique known as living radical polymerization. Linear-type PMB was also synthesized using ATRP. The effects of the polymer architecture on cytotoxicity and cellular uptake were investigated. Both 4armPMB and LinearPMB were successfully synthesized, and these polymers were verified to be water soluble. Pyrene fluorescence in the polymer solution indicated that the architecture had no effect on the behavior of the polymer aggregates. In addition, these polymers caused no cytotoxicity or cell membrane damage. The 4armPMB and LinearPMB penetrated into the cells after a short incubation period, with similar rates. In contrast, the 4armPMB showed a quicker back-diffusion from the cells than that of LinearPMB. The 4armPMB showed fast cellular internalization and exiting behaviors.