Herein, we report the synthesis and characterization of a novel class of polymer composites based on onion-like carbons (OLCs)-silicon diimide by a salt-free polycondensation reaction. The pyridine-catalyzed polymerization reaction was carried out in the presence of various contents (0.1, 0.5, 1, and 2 wt%) of carboxyl-functionalized OLCs in argon atmosphere to provide composites with well-dispersed and covalently incorporated 0D nanocarbons throughout the 3D matrix of silicon diimide polymer. A strong dependency of the optical properties (UV absorbance and the photoluminescence spectra) on the content of functionalized OLCs incorporated within the polymer matrix was observed. The novel polymer composites are suitable precursors for the design of advanced and multifunctional 0D-nanocarbon–containing Si3N4-based ceramic nanocomposites.