Biodegradable polymers are a global topic of interest, particularly bio-based polymers such as poly(lactic acid) (PLA). PLA is also commonly blended with other polymers to improve its properties and expand its applications. When used as a compatibilizer, PLA grafted with maleic anhydride has been shown to improve the mechanical properties of PLA blends with starch and in PLA/layered-silicate nanocomposites. In this research, the grafting of maleic anhydride to granulated PLA was demonstrated in a supercritical carbon dioxide (scCO2) reaction medium. This process is preferred over alternative grafting methods because most polymers swell significantly in scCO2, the maleic anhydride monomer and benzoyl peroxide initiator can be dissolved by scCO2, and the simple separation step of depressurization removes both spent initiator and excess monomer. NMR spectroscopy and FTIR spectroscopy were used to verify grafting and to determine the achievable graft level.