In this study, we propose the synthesis of silver (Ag) nanoparticle decorated TiO2 nanoflowers (TNFs) using a polymeric conformal thin film of 3,4-dihydroxyphenyl-L-alanine (PLDOPA). The size, as well as interparticle distance of the decorated Ag nanostructures, were tuned by simply changing the number of supplied Ag ions. The resultant nanocomposite system (TNF@PLDOPA@Ag) showed unique SERS activity with high reproducibility and an enhancement factor of 4.2 × 107. Also, as an ideal photocatalytic system, the TNF@PLDOPA@Ag enabled the catalytic degradation of malachite green under visible light irradiation. Display omitted •Silver nanostructure decorated TiO2 nanoflowers (TNFs) were fabricated using a thin film of the polymer of L-DOPA.•The PLDOPA layer enabled the reduction of silver ions and adsorption of the resultant silver nanostructures.•The size and interparticle distance of the deposited Ag nanostructures were manipulated via the number of supplied Ag ions.•The TNF@PLDOPA@Ag nanosystem showed unique SERS activity with high reproducibility and an enhancement factor of 4.2 ×107.•The TNF@PLDOPA@Ag system enabled the catalytic degradation of malachite green under visible light irradiation.