The Westerlands, also known simply as the West, is a realm of Westeros.

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.

The seat of the Lannister's power for generations, Casterly Rock, for the first time in its history, fell to an invading army during the Last War. Utilizing 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.

The Unsullied would later abandon Casterly Rock to participate in battles in the North, and with the destruction of the remaining Lannister forces during the Battle of King's Landing and the death of Cersei Lannister, lordship of both House Lannister and Casterly Rock was passed on to Tyrion.

Bastards born in the Westerlands are given the surname Hill.