Polydimethyl siloxane (PDMS) CuO nanocomposites (NCs) were prepared using plain CuO and cetyltrimethyl ammonium bromide (CTAB) capped (0.05 & 1.0M) CuO nanoparticles as nanofillers. Incorporation of plain CuO and CuO-1M capped NP as nanofillers into PDMS altered their surface roughness and imparted biocidal activity by release of metal ions. CTAB capping increased the zeta potential of synthesized nanoparticles with 0.05M CTAB yielding monodispersed CuO-NP with enhanced broad spectrum antifouling activity in PDMS matrix. Nanocomposites were tested for their efficacy against inhibition of bacterial (in vitro) and diatom adhesion (in situ) in laboratory and coastal waters respectively. PDMS-CuO-CTAB (0.05M) capped nanoparticles showed significant (p<0.05) antibiofilm property against Staphylococcus lentus and Vibrio natriegens. Diatoms extensively colonized plain PDMS surfaces (1.2 × 10³ cm^–2), whereas PDMS-CuO, PDMS-CuO-0.05M CTAB, PDMS-CuO-1M CTAB NCs showed 1.20 × 10³, 4.86 × 10² and 5.82 × 10² cells cm^–2 respectively. Field exposure trials showed reduction in fouling biomass from 9 Kg m^–2 (on plain PDMS) to 0.6 Kg m^-2 on PDMS-CuO (0.05M CTAB) NC after 90 days of exposure in coastal waters. Percentage surface area coverage of fouling organisms also reduced significantly (p<0.05) from 98.3 ± 0.6% on plain PDMS to 36.8 ± 2.34% on PDMS-CuO-(0.05M CTAB) nanocomposite. Overall, PDMS-CuO-(0.05M CTAB) nanocomposite exhibited broad spectrum antifouling activity against different levels of fouling organisms and offers a potential antifouling surface.