Louisa Reef (Brunei Malay: Terumbu Semarang Barat Kecil; Mandarin Chinese: 南通礁; pinyin: Nántōng Jiāo) is a coral atoll in the southern reaches of the Spratly Islands. The reef is oval in shape, approximately 1.5 km east to west and 0.5 km north to south. While most of Louisa Reef is submerged, some portions emerge at low tide and a few small rocks remain above water even at high tide. The atoll rises as deeply undercut walls and steep slopes from very deep water with extensive stony and soft corals. A navigation beacon is at its southwest point and the highest visible rock is at the southeast end.