In this paper, we investigate the role of cellulose nanofibres (CNFs) in the supercritical foaming process of polylactic acid (PLA). The supercritical foaming process of nanocomposites is determined by the nucleating efficiency of the nanoparticle and rheological properties of the polymer melt. Here, the nucleating efficiency of surface acetylated CNFs was compared with that of native CNFs and different CNF loadings were introduced into the PLA matrix forming more or less stiff networks. We characterized the tensile and shear properties of the nanocomposite melts and studied the surface tension at the nucleating site. It was observed that the introduction of CNFs significantly improved the morphology of PLA foams. Both the amount of CNFs and surface acetylation contributed to the reduction in cell size and increase in cell density. The nucleating effect of the CNF surface was clearly evidenced and revealed to be enhanced by surface acetylation. Improved morphology in nanocomposite foams was therefore explained by the interplay of the nucleating and rheological effects of CNFs during the foaming process. Finally, the potential of CNFs to generate nanoporous structures via supercritical foaming was discussed.