The preparation of polypropylene (PP)/microcrystalline cellulose (MCC) composites and their applicability for material extrusion additive manufacturing (ME-AM) is reported. MCC is modified by grafting onto its surface with different silanes, in particular perfluorooctyltriethoxysilane, n-octyltriethoxysilane (OTS), or aminopropyltriethoxysilane. The efficacy of the surface modification is confirmed by attenuated total reflectance and X-ray photoelectron spectroscopy. The affinity of the modified MCC to the polar PP matrix is investigated by direct melt-compounding, and the applicability of the resulting composites for material ME-AM is accessed by fabrication of filaments and evaluating the relevant property requirements. The surface modification of the MCC improves their dispersibility in PP and enhances the mechanical properties of the composites. Moreover, the OTS-modified MCC shows the best reinforcement, good surface finish of the filament, and flawless printability.