Cuarteron Reef, also known as Calderon Reef, is a reef at the east end of the London Reefs in the Spratly Islands of the South China Sea. It is occupied and controlled by China (PRC), and also claimed by the Philippines, by Vietnam and Taiwan (ROC). The reef is 3 nautical miles long and has an area of 8 square kilometres (3.1 sq mi).

The London Reefs are located between 8°49′N 112°12′E and 8°56′N 112°52′E in the Spratly Islands of the South China Sea.