Perovskite nanoparticle-based nanocomposite thin films strictly tailored using unconventional layer-by-layer (LbL) assembly in organic media for piezoelectric nanogenerators (NGs) are demonstrated. By employing sub-20-nm BaTiO3 nanoparticles stabilized by oleic acid ligands (i.e., OA-BTONPs) and carboxylic acid (COOH)-functionalized polymers, such as poly(acrylic acid) (PAA), the resulting OA-BTONP/PAA nanocomposite multilayers are prepared by exploiting the high affinity between the COOH groups of PAA and the BTONPs. The ferroelectric and piezoelectric performance of the (PAA/OA-BTONP)n thin films can be precisely controlled by altering the bilayer number, inserted polymer type, and OA-BTONP size. It is found that the LbL assembly in nonpolar solvent media can effectively increase the quantity of adsorbed OA-BTONPs, resulting in the dramatic enhancement of electric power output from the piezoelectric NGs. Furthermore, very low leakage currents are detected from the (PAA/OA-BTONP)n thin films for obtaining highly reliable power-generating performance of piezoelectric NGs.