Polluted sea water is a common problem in coastal cities, but its decontamination remains a big challenge due to the existence of salt ions, which may invalidate many prevailing treatment methods originally developed for waste fresh water. This work presents the first attempt to decontaminate the polluted sea water by synergizing two separate processes: photocatalysis and ozonation. For low cost and human safety, the ozone concentration is kept to be < 60 ppm in the pumping gas (0.026 ppm in solution). Mechanism studies and experimental comparisons show that the photocatalysis, the ozonation and the photocatalytic ozonation (PCO) all have lower efficiencies in sea water than that in fresh water, and the PCO is always more efficient than only the photocatalysis or only the ozonation. More specifically, the PCO has a reaction rate constant about 23 times higher than only the ozonation. In addition, the sea water shows a positive synergistic effect of photocatalysis and ozonation and reaches the maximum when the pumping gas has an ozone concentration of 50 ppm in gas. In contrast, the fresh water shows a negative synergistic effect. Mechanism study using experiments infers that the dissolved ozone can enhance and stabilize the photocurrent by rapidly scavenging the photoelectrons. This work may pave the way to practical applications of sea water decontamination with high efficiency and low cost.