Southwest Cay, also known as Vietnamese: Đảo Song Tử Tây; Pugad Island (Filipino: Pulo ng Pugad, lit. 'Island of Nest'); Mandarin Chinese: 南子島/南子岛; pinyin: Nánzi Dǎo, is an island on the northwestern edge of the Spratly Islands in the South China Sea. It is part of the atoll North Danger Reef, and just 1.75 miles (2.82 km) southwest of Northeast Cay. With an natural area of 12 hectares (30 acres), it is the sixth largest of the naturally occurring Spratly Islands, and the second largest of the Vietnamese-occupied islands. Southwest Cay has the archipelago's highest point, at 4 meters above sea level. It was once a breeding place for birds, and was covered with trees and guano; export of guano had been carried out "on a considerable scale".

The island is also claimed by China (PRC), Taiwan (ROC), and the Philippines.

In 1933, the French Government took possession of the Spratly Islands including Southwest Cay and Northeast Cay. Three ships, the Alerte, the Astrolabe and the De Lanessan took part in the expedition. The following quotations are from an account given by H. Cucherousset in L'Eveil economique de l'Indochine (No. 790 of May 28, 1933):

After possession had been taken, the French Ministry of Foreign Affairs published the following notice in the French Journal Officiel dated 26 July 1933 (page 7837):

In 1939, Japan occupied the islands and remained there until the end of World War II.

In 1956, France transferred the islands to South Vietnam.^{[citation needed]}

In 1959, the South Vietnamese Government renamed the Song Tử islands (meaning twins after their resemblance to each other) "Tây" (meaning West) and "Đông" (meaning East) respectively. They were annexed to Phước Tuy Province, South Vietnam.^{[citation needed]}

In 1961, the two South Vietnam Navy cruisers, the Van Kiep and the Van Don, landed on the islands of Song Tử Tây (Southwest Cay).^{[citation needed]}