The tribological behaviour of innovative water-based nanolubricant containing TiO2 nanoparticles (NPs) on ferritic stainless steel (FSS) 445 was investigated 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 lubricant containing 4.0 wt% TiO2 exhibits excellent tribological properties, including the lowest COF and the strongest anti-wear ability on the applied balls under all lubrication conditions. The proposed lubrication mechanisms are primarily attributed to the formation of tribofilm and ball-bearing effect of the TiO2 NPs, respectively, on the clean and pre-oxidised disks. In particular, the transfer of ball material onto the pre-oxidised disk surface promotes the formation of Fe-rich oxides on disk by tribo-sintering, which facilitates the decrease of the ball wear.