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 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.