Provided are a water treatment system and a water treatment method capable of downsizing a facility and inexpensively producing freshwater. A water treatment system includes: a first reverse osmosis treatment device for separating wastewater into desalinated permeate water and concentrated wastewater; a mixing unit for mixing the separated concentrated wastewater and intake seawater; a second reverse osmosis treatment device for separating mixed water obtained by mixing the concentrated wastewater and the seawater into desalinated permeate water and concentrated mixed water; and a return pipe for returning a portion of the separated concentrated mixed water to the mixing unit. The water treatment method includes the following steps: mixing intake seawater with wastewater; separating the mixed water into permeate water and concentrated mixed water; re-mixing a portion of the concentrated mixed water with the mixed water; and producing freshwater by re-separating the re-mixed water into permeate water and concentrated mixed water.