The Westerlands is one of the constituent regions of the Seven Kingdoms. It was formerly a sovereign realm known as the Kingdom of the Rock before the  Targaryen conquest. The Westerlands are ruled from the castle of Casterly Rock by House Lannister. It is one of the smaller regions of the Seven Kingdoms geographically but has one of the larger populations. It is also immensely rich in natural resources, particularly metals. Predominantly mountainous, the hills of the Westerlands are riddled with veins of gold and silver, the mining of which has made the Lannisters and their bannermen immensely rich; the abundance of iron also allows them to easily forge weapons and military equipment. While the Lannister armies are not as huge as those of the Reach, they are the best-equipped in the realm with heavily-armored soldiers and cavalry. Bastards born in the Westerlands are given the surname Hill. People of the Westerlands are collectively known as Westermen and are referred to, colloquially, as "lions" in reference to the sigil of House Lannister. Colloquially, Westermen are also referred to as Lannisters.

Once the seat of the Lannister's power, Casterly Rock has for the first time in its history fallen to a foe. Utilising a secret passageway constructed by Tyrion Lannister, the Unsullied armies of Daenerys Targaryen infiltrated and captured the castle. Though the Lannisters had deliberately left the Rock underdefended so as to focus their attention on capturing the Tyrell stronghold of Highgarden and were intending to take it back later, their armies were in turn smashed by Daenerys at the Battle of the Goldroad, thus leaving the Unsullied in control of the castle. Despite Casterly Rock being captured by the Unsullied, The Westerlands still remained under complete Lannister control, as Jaime Lannister was seen discussing troop and supply movements in the Westerlands with his generals. The Lannister forces in King's Landing were then destroyed during the Battle of King's Landing, with the lordship passing on to Tyrion after the death of Cersei and Jaime Lannister.

The Westerlands border the Riverlands to the east and the Reach to the south. The Goldroad links Lannisport on Westeros' west coast with the capital city of King's Landing on the east coast, passing through the Reach, skirting south of the Riverlands. The Searoad, hugging the coast, links Lannisport with the capital of the Reach at Highgarden as well as the major city of Oldtown further south. Predominantly mountainous, the Westerlands are well-defended from external attack. There is only one major pass through the mountains in the east of the Westerlands, near the headwaters of the Red Fork of the Trident River. Thus there is only one narrow and predictable path that armies moving between the Westerlands and Riverlands can normally take and it is guarded by the castle of Golden Tooth.

Notable places shown in the series are: