Tarth is an island in off the coast of eastern Westeros in the Narrow Sea. It is ruled by Lord Selwyn of House Tarth.
The Island is referred to as the "Sapphire Isle" due to the blue of the surrounding waters. Brienne of Tarth is the only daughter and thus heir to Evenfall Hall, the seat of the Tarths. Brienne tells Jaime that her upbringing on the island taught her to handle a boat from a very young age.