Cuarteron Reef, also known as Calderon Reef (Mandarin Chinese: 華陽礁/华阳礁; pinyin: Huáyáng Jiāo; Filipino: Bahura ng Calderon; Vietnamese: Đá Châu Viên; Malay: Terumbu Kalderon), 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) (as part of Sansha), and also claimed by the Philippines (as part of Kalayaan), by Vietnam (as part of Truong Sa) and Taiwan (ROC). The reef is 3 nautical miles (5.6 km; 3.5 mi) long and has an area of 8 square kilometres (3.1 sq mi) (800 hectares (2,000 acres)).

On 12 July 2016, the tribunal of the Permanent Court of Arbitration concluded that Cuarteron Reef contains, within the meaning of Article 121(1) of the Convention, naturally formed areas of land, surrounded by water, which are above water at high tide. However, for purposes of Article 121(3) of the convention, the high-tide features at Cuarteron Reef is rocks that cannot sustain human habitation or economic life of their own and accordingly shall be entitled to 12nmi of territorial sea measured from its baseline but have no exclusive economic zone or continental shelf.

Prior to 2016, Carteron reef had a supply platform and a reef fortress. In 2015 the Philippine government released what it claimed were photos of a six-story facility being built by the Chinese government on the reef. Also in 2015, China completed the construction of a 50 metres (160 ft) high lighthouse on the reef. By late 2015, China had expanded the reef's usable area to 23 hectares (57 acres), reportedly including in the process two helicopter pads, and possibly gun, radar and missile emplacements. As of 2016, China was reportedly building a high-power radar installation on the reef.

In late 2016, photographs emerged which suggested that Cuarteron Reef has been armed with anti-aircraft weapons and a CIWS missile-defence system.

• Great wall of sand
• Nine-dash line

• Asia Maritime Transparency Initiative Island Tracker