Oil/water separation has become a worldwide subject and challenge because of the increasing amount of oily water generated by oil spills and other oil-related industrial or daily activities. Superwetting (superhydrophilic–underwater superoleophobic, superhydrophobic–superoleophilic and superhydrophobic–superoleophobic, etc.) nanomaterials offer an encouraging opportunity for people to develop brand new, energy-efficient and process-simple strategies to treat oily water due to their selective behaviour for oil and water. Theoretically, superwetting nanomaterials can be constructed by combining surface chemical composition and micro/nanostructured surface roughness. In this chapter, we will introduce how to construct superwetting nanomaterials based on the theoretical principle in detail. Recent advanced works on separation of free and emulsified oil/water mixtures by superwetting nanomaterials including superwetting absorbing nanomaterials, mesh- or textile-based superwetting films, polymer-dominated superwetting membranes and 1D nanomaterial-based superwetting ultrathin films are also discussed in detail. Finally, a summary of the remaining challenges and a perspective for future works on oil/water separation are described. These superwetting nanomaterials hold promising potential for treating oily wastewater in large quantities, as an alternative to traditional separation techniques in the near future.