Palladium nanoparticles (PdNPs) supported on cellulose nanocrystals (CNXL) were synthesized in a single step from Pd(hexafluoroacetylacetonate)2 (Pd(hfac)2) in subcritical and supercritical carbon dioxide. CNXLs acted as both the reducing agent and support material for the obtained nanoparticles. Dry Pd nanoparticles supported on the cellulose nanocrystals (PdNP@CNXL) were obtained by simply venting the CO2 and were characterized by FT-IR, X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). The results show that the Pd nanoparticle diameters varied between 6 and 13 nm with varying pressure (240-2200 psi), reaction time (2-17 h), and weight ratio of the precursor Pd(hfac)2 to CNXL (1-4% w/w). Particles with diameters above 13 nm appeared not to remain attached to the CNXL surface. Reaction conditions also affected the Pd loading in the final PdNP@CNXL composite. Finally, the PdNP@CNXL composites were shown to be effective catalysts for carbon-carbon bond formation in the Mizoroki-Heck cross-coupling reaction, in line with other reports.