Chitosan-decorated copper nanopartides <2 nm in size were synthesized in large scale (5.2 g, 98%) by a green chemical reduction method. The reduction of copper(II) salt by L-ascorbic acid (green reducing agent) in water medium in the presence of chitosan (green capping agent) at 85 C resulted in the formation of chitosan-decorated copper nanopartides (CDC NPs). The chitosan-decorated copper nanopartides were characterized by spectral (diffuse reflectance spectroscopy, Fourier transform infrared spectroscsopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, and high-resolution transmission electron microscopy) and thermal (thermogravimetirc analysis) studies. The CDC NPs were mixed with polyurethane clear, polyurethane white, and acrylic emulsion paint and coated on mild steel, cement slab, and wood panels. The measurement of water contact angle and hydration free energy of uncoated and CDC nanopaint-coated panels indicated the extent of hydrophilicity. The antifouling activities of CDC NP-coated panels were examined against marine (Amphora) and green (Arthrospira and Chlorella) algae. The amount of copper leached from the surface of CDCNP-coated panels is near zero, which is confirmed by inductively coupled plasma spectrometry measurements. The CDC NPs show similar to 80-95% antifouling efficiency against the growth of algae.