Transparent BiFeO₃/PMMA (PMMA = poly(methyl methacrylate)) nanocomposites with a thickness up to 40 μm are obtained via solution film casting of BiFeO₃ nanoparticle dispersions. The multiferroic BiFeO₃ nanoparticles are obtained in a space confined crystallisation process in a porous carbon matrix and subsequent removal of the matrix. The average crystallite size ranges from 46 nm to 106 nm for materials synthesised at 803 K and 903 K respectively, but, at higher temperature, significant amounts of side products are detected by means of X-ray powder diffraction. The nanoparticles are multiferroic with a magnetic coercivity of 1.88 A ^m-1 at 400 K and a polarisation of up to 1.04 μC/cm² at 65 kV/cm (298 K). Surface functionalisation using oleic acid allows the integration into PMMA films with a high transmittance.