Substrate roughness is a critical characteristic of a protective coating interface that, if well-controlled, can allow one to significantly enhance the mechanical, tribological as well as the corrosion properties of metallic substrates. In the present work, we investigate the effect of the substrate surface roughness on the tribocorrosion behavior of DLC coatings on SS304 steel in a 3.5 % NaCl electrolyte. The surface of the 304-type stainless steel substrates with different roughness levels, systematically varying between 1.5 µm and 0.1 µm, was produced using the abrasive polishing process in which pads with different grit sizes were used. Subsequently, a 2 µm layer of hydrogenated diamond like carbon (DLC) prepared using the plasma enhanced chemical vapor deposition (PECVD) technique was deposited. The tribological tests were performed using a reciprocating tribometer with an alumina (Al2O3) ball as a counterpart material. For the tribocorrosion tests, the tribological contact between the sample and the Al2O3 ball was immersed in a 3.5 % NaCl electrolyte chosen to mimic the sea water, and the corrosion potential was recorded as a function of time using a three-electrode cell. The results show that the application of the DLC layer allowed one to decrease the coefficient of friction (µ) of the SS304 substrates from 0.6 down to < 0.05, and to significantly increase its wear resistance under dry tribological conditions irrespective of the surface roughness of the substrate. In particular, µ of the DLC/SS304 coatings with a substrate surface roughness higher than 0.4 µm increased and stabilized above 0.1 due to debris from the worn asperities. In the case of coatings with a substrate surface roughness lower than 0.4 µm, µ further decreased down to about 0.05. DLC/SS304 coatings with substrate surface roughness below 0.4 µm show a better tribocorrosion behavior compared to the substrates. Its wear resistance is about two orders of magnitude higher. The degradation mechanisms of coatings are discussed in detail.