Summer Sea

The Summer Sea is a warm body of water located to the south of both Westeros and Essos, separating those continents from the southern landmass of Sothoryos. It is separated from the Narrow Sea by the islands known as the Stepstones and from the Jade Sea by the narrow Straits of Qarth. The Summer Isles lie on this ocean, some distance south of Westeros. Slaver's Bay is an inlet of the Summer Sea.

In the books
In the Song of Ice and Fire novels, the Summer Sea is one of the largest bodies of water in the world, outstripped only by the Sunset Sea lying to the west of Westeros.