By adding poly(ethylene oxide) (PEO) to the growth medium of Acetobacter xylinum, finely dispersed bacterial cellulose (BC)/PEO nanocomposites were produced in a wide range of compositions and morphologies. As the BC/PEO w/w ratio increased from 15:85 to 59:41, the cellulose nanofibers became smaller but aggregated in larger bundles, indicating that PEO mixed with the cellulose on the nanometer scale. Fourier transform infrared spectroscopy suggested intermolecular hydrogen bonding and also preferred crystallization into cellulose I_β in the BC/PEO nanocomposites. The fine dispersion of cellulose nanofibers hindered the crystallization of PEO, lowering its melting point and crystallinity in the nanocomposites although remaining bacterial cell debris also contributed to the melting point depression. The decomposition temperature of PEO also increased by approximately 15 °C, and the tensile storage modulus of PEO improved significantly especially above 50 °C in the nanocomposites. It is argued that this integrated manufacturing approach to fiber-reinforced thermoplastic nanocomposites affords a good flexibility for tailoring morphology and properties. These results further pose the question of the necessity to remove bacterial cells to achieve desirable materials properties in biologically derived products.