Provided is a water treatment method with which, in a reverse osmosis membrane treatment conducted on water being treated that contains chlorine-based oxidants and/or bromine-based oxidants, or on water being treated that contains iodide ions, it is possible to suppress any deterioration in a reverse osmosis membrane and to suppress the formation of slime. This water treatment method includes a reverse osmosis membrane treatment, the water treatment method involving: adding, to water being treated that contains chlorine-based oxidants and/or bromine-based oxidants, 1 mol or more of iodide ions per mole of the free chlorine content and free bromine content of the water being treated; or adding, to water being treated that contains iodide ions, a chlorine-based oxidant and/or a bromine-based oxidant such that the free chlorine content and free bromine content reach 1 mol or less per mole of iodide ions in the water being treated.