The tribological behaviour of innovative TiO₂ nano-additive water-based lubricant on ferritic stainless steel (FSS) 445 was characterised on a ball-on-disk tribometer. Non-oxidised (clean) and pre-oxidised FSS 445 disks with original rough surfaces were both applied in the tribological tests at room temperature. The results show that the water-based nanolubricants with concentrations of 0.4–8.0 wt% TiO₂ can significantly reduce the coefficient of friction (COF) on the two types of disks. The 4.0 wt% TiO₂ lubricant exhibits optimal tribological properties, including the lowest COF and the strongest anti-wear ability on the applied balls under all lubrication conditions. The lubrication mechanisms are primarily ascribed to the formation of tribofilm and the ball-bearing effect of the TiO₂ nanoparticles, respectively, on the two different disks.