Rosemary extraction residues (RERs) are unused biomass released from the supercritical CO2 extraction of rosemary essential oil. To reduce waste, RERs were used for the simple preparation of copper nanoparticle-loaded biochar (CuNPs/BC). One-pot pyrolysis of cupric acetate-impregnated RERs created zero-valent copper nanoparticles (CuNPs) with an average particle size of 15 ± 2 nm within the biochar (BC) matrix. Accordingly, the CuNPs/BC nanocomposite was applied for peroxydisulfate (PDS) activation in Ponceau 4R (P4R) decolorization. At 25 oC, pH 6.0, and 2.00 mM PDS, 1.00 g/L CuNPs/BC decolorized 99.3% P4R (20 ppm) after 120 min. Additionally, P4R decolorization was effective in a broad range of initial pH values (3.0–9.0). Altogether, RER-derived CuNPs/BC proved high potential for PDS activation towards P4R decolorization.