Cellulose nanoparticles (CNPs) were prepared from jute fiber by acid hydrolysis followed by high‐speed homogenization. The CNPs were used as fillers in the production of polymethylmethacrylate (PMMA) nanocomposites by in situ suspension polymerization technique. The suspension polymerization of MMA was carried out in the presence of CNPs, which were dispersed in water medium and in situ PMMA/cellulose nanocomposite granules were formed. PMMA polymer, without any filler, was also prepared by similar suspension polymerization technique. PMMA and PMMA/cellulose nanocomposite films were prepared by solution casting method. Viscosity average molecular weights of neat PMMA and the PMMA extracted from PMMA/cellulose nanocomposite granules were determined by viscometric method and average molecular mass of PMMA extracted from PMMA/cellulose nanocomposites was found to be reduced than that of neat PMMA. Attenuated total reflectance Fourier transform infrared spectroscopy was performed to find out any chemical interaction between polymer matrix and the CNPs. X‐ray diffraction study and differential scanning calorimetry were done to investigate the structures of the nanocomposite films and the glass transition temperature was found to be lower in the nanocomposite than that in the virgin polymer. Field emission scanning electron microscopy and atomic force microscopy were done to examine the morphology of the films. Such an in situ suspension polymerization technique for the preparation of PMMA/cellulose nanocomposites can be very useful to prepare tailor‐made materials.