This article presents a study on the foam processing of sustainable biocomposites made of polyl(actic acid) (PLA) and functionalized microfibrillated cellulose (MFC) using water as a foaming agent. MFC contains moisture and can be used as a water carrying agent in a 'green' foaming process. The quantity of water, that is, of foaming agent, was adjusted by chemically modifying the fibrils, changing their surface hydroxyl groups, and thus tailoring their hygroscopicity. Functionalized carboxymethylated and acetylated MFC were processed with PLA in a two-step microextruder and various foams were produced. The thermal behavior of the MFC was fully investigated to determine the amount of available foaming agent. Densities and morphologies of resulting foams were correlated to processing temperatures and to water contents ranging from 3.7 to 98 wt%. A low processing temperature and MFC water content of 9 wt% provided uniform foams with the lowest densities. However, too high water content showed poor foaming efficiency and high densities. The obtained foams were compared with foam processed with a chemical foaming agent, used alone or in combination with the MFC.