The Reach is one of the administrative regions of the Seven Kingdoms, and was formerly a sovereign nation known as the Kingdom of the Reach before the  Targaryen conquest. Geographically, it is one of the larger regions of the Seven Kingdoms, ranking as the second largest in area, behind only the vastness of the North. It has been ruled by House Tyrell since the Conquest, and earlier than that by the Kings of House Gardener. It is the most fertile part of Westeros, blessed with vast, blooming fields of crops and flowers, and numerous and well-populated villages and towns; it traditionally helps supply other less fertile parts of the Seven Kingdoms (most notably King's Landing) with grain, fruit, wine and livestock. The Reach is also the most heavily-populated part of Westeros and is one of the richest, second only to the Westerlands, ruled by House Lannister. However this claim is suggested to be rather outdated as Lannister gold mines—the source of their wealth—have dried out, and have been weakened by the War of the Five Kings, thus making the Reach actually the wealthiest region. Due to this, the Tyrells could traditionally field the largest army in the Seven Kingdoms, almost as large as the armies from two of the other populous kingdoms combined. This vast supply of manpower, land, and food production gives the Reach enormous strategic value during the War of the Five Kings.

The Reach withdrew from the sovereignty of the Iron Throne due to the deaths of virtually all its liege lords in the destruction of the Great Sept of Baelor, an act perpetrated by Cersei Lannister, who quickly became Queen due to the resulting suicide of King Tommen Baratheon. The Reach was ruled from the castle of  Highgarden by House Tyrell until the castle was sacked by Lannister and rebel forces in the Sack of Highgarden. After the battle and the ultimate death of Lady  Olenna Tyrell, House Tarly was made Warden of the South for its betrayal of its liege lords. Thus the Reach was presumably briefly controlled by the Lannister-Tarly houses, before their host was smashed in the Battle of the Goldroad, leaving the region in an ambiguous status and likely a power vacuum. Ser Bronn of the Blackwater offered to join Jaime and Tyrion Lannister with the forces of Queen Daenerys Targaryen should he be named Lord of Highgarden, a deal to which Tyrion reluctantly agreed upon. Bastards born in the Reach are given the surname Flowers. People of the Reach are collectively known as Reachmen.

The Reach is watered by the immense River Mander and its many tributaries. The borders of the Reach are held as the Sunset Sea to the west and southwest, the Red Mountains of Dorne to the southeast, the hills of the Westerlands to the northwest and the Blackwater Rush to the far northeast. The northern Red Mountains also form the Reach's border with the Stormlands: known as the Dornish Marches, and currently possessed by the Stormlands, these rugged mountains have for centuries been the site of shifting border wars between the Reach, the Stormlands and Dorne.

Notable places shown in the series are: