| Disputed: | yes |
| Louisa Reef | |
| Type: | reef |
| Other Names: | Terumbu Semarang Barat Kecil (Malay) 南通礁 Nántōng Jiāo (Chinese) |
| Pushpin Map: | Spratly Islands relief |
| Location: | South China Sea |
| Archipelago: | Spratly Islands |
| Country1 Admin Divisions Title: | EEZ |
| Country1 Admin Divisions: | Brunei zone |
| Country2 Admin Divisions Title: | City |
| Country2 Admin Divisions: | Sansha, Hainan |
| Country3 Admin Divisions Title: | Municipality |
| Country3 Admin Divisions: | Cijin, Kaohsiung |
| Country4 Admin Divisions Title: | District |
| Country4 Admin Divisions: | Trường Sa, Khánh Hòa |
Louisa Reef (Brunei ms|'''Terumbu Semarang Barat Kecil'''|lit=; Mandarin) is a coral reef in the southern reaches of the Spratly Islands. The reef is oval in shape, approximately 1.5 km (1 mile) east to west and 0.5 km (500 yards) 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 reef rises as deeply undercut walls and steep slopes from very deep water with extensive stony and soft corals. A navigation beacon built by Malaysia is at its southwest point and the highest visible rock is at the southeast end.
Safe anchorage is difficult to find, depending on the wind and currents, but there are sandy areas at the northwest and southwest ends. The nearest reliable shelter for boats is the lagoon at Swallow Reef, 128 km (80 miles) to the northeast.
, the shoal was claimed by Brunei, China, and Taiwan. Malaysia previously claimed Louisa Reef and operated a small obelisk-like concrete navigational light beacon there. However, Malaysia has apparently dropped its claim in deference to the claim of neighbouring Brunei that the reef lies on its continental shelf.[1]
