We reported here the first considered study of the optical and magnetic properties of biochar (BC)-modified CuO nanoparticles (NPs) for (CuO)x/(BC)1-x nanocomposites (NCs) with various x (0.00 ≤x ≤1.00). The BC and NC of x =0.05 confirm amorphous structures, whereas the CuO NPs and other NCs reveal monoclinic structures. Adding CuO NPs to BC changed the morphology of particles from aggregates to spherical and diminished the particle size and crystallite sizes. The EDX spectra exhibit characteristic peaks corresponding to the starting elements C, Cu, and O. The NCs of x =0.10 exhibit the highest value of oxygen weight percent (37.6 %), whereas the BC shows the lowest value of 2.9 %, and meanwhile the other samples. In general, the NCs show higher optical absorbance than the BC or CuO NPs. In addition, all samples show a single transition of unique energy gaps (Eg) of 2.95, 2.9, 3.2, 2.65, 2.7, 2.75, and 2.7 eV for the BC, NCs, and CuO NPs, respectively. As compared to NCs, the BC and x =0.50 and 1 (CuO NPs) samples reveal lower dielectric loss. The carrier density (N/m*) of BC is 0.86 ×1054 (g.cm3) 1, but it decreases with an increase of x to 0.20, and then enhances till reaches 2.46 ×1055 and 7.38 ×1055 (g.cm3) 1 for x =0.50 and 1, respectively. The single oscillator energy (Eo) is 8.73 eV for the BC, which is higher than the dispersion energy (Ed) of 3.65 eV, and the vice is true for the CuO NPs and NCs, where Ed >Eo. The BC reveals diamagnetic behavior, but it changed to ferromagnetic followed by diamagnetic for x = 0.05 NCs. Interestingly, the behavior changed to considerably ferromagnetic for x =0.10 and 0.20 NCs, but above that it becomes paramagnetic. The coercive field (Hc) of BC increased from 66 to 155.8 G with increasing x to 0.20, but above that it decreases to be lower than those of BC (66 G) and CuO NPs (44.2 G). The BC exhibited a small magnetic anisotropy (Keff) of 0.02 (emu⋅G/g), but the increase of x up to 0.0.05 was associated with the gradual development of Keff to 10.7 (emu⋅G/g), which is lower than that of CuO NPs (23.98 emu⋅G/g), whereas a further increase of x resulted in the Keff decreasing. These characteristics make some of the NCs more favorable than BC or CuO NCs for the devices of light-emitting diodes, solar cells, magnetic data storage, and spintronics.