Liguasan Marsh, also known as Ligawasan Marsh, is the largest swamp and marsh area in south-central Mindanao, Philippines. It spans approximately 2,200 square kilometers (850 sq mi) across the provinces of Cotabato, Maguindanao del Norte, Maguindanao del Sur, and Sultan Kudarat. About 300 square kilometers (120 sq mi) of the marsh is designated as a game refuge and bird sanctuary.

The marsh is part of a complex river system featuring freshwater lakes, ponds, and marshes in the Mindanao River basin. It actually consists of two adjoining basins: Liguasan Marsh and Libungan Marsh, each with distinct water regimes. Approximately 5,000 hectares of old-growth forest remain within the marsh, contributing to its rich biodiversity.

Liguasan Marsh is recognized as an Important Bird Area (IBA) due to its significance for bird species conservation. Several threatened species have been recorded in the marsh, including the Philippine eagle, though recent information on their status is limited. The relatively extensive lowland forests in the marsh likely support populations of other threatened and restricted-range birds from the Mindanao and Eastern Visayas Endemic Bird Area.

Liguasan Marsh is also an important wetland, supporting resident or non-breeding populations of various waterbirds such as herons, egrets, rails, shorebirds, and ducks. Among these species are a Mindanao-endemic subspecies of the Little grebe (Tachybaptus ruficollis cotabato) and the Comb-crested jacana, the latter being observed only in this region within the Philippines.

In addition, it is one of the last strongholds for both of the only native crocodilians in the country: the critically endangered and endemic Philippine Crocodile and the Saltwater Crocodile.

Liguasan Marsh plays a crucial role in flood mitigation by absorbing excess water from the Cotabato River Basin, which prevents downstream flooding and sustains farming in the lowlands. The marsh is also a key livelihood source for more than 112,000 Maguindanaon families, who rely on fishing during high water periods and agriculture during the dry season.

The marsh also faces significant threats from illegal fishing, deforestation, and land conversion for agriculture, which have caused habitat loss and degradation.[10] Research conducted between 2015 and 2017 identified Common snakehead, Eurasian carp, Rohu, and Nile tilapia as the most commonly caught species in the area, with overfishing being a major concern due to the capture of immature individuals.

Efforts to manage and conserve Liguasan Marsh have been ongoing for decades. In 1941, the area was declared a Game Refuge and Bird Sanctuary by Forestry Administrative Order No. 19, and it was later included under the National Integrated Protected Areas System (NIPAS) in 1992. More recently, a measure in the Bangsamoro Parliament, BTA Bill No. 310 (the LigMarsh Act of 2021), aims to consolidate efforts to protect and manage the marsh. The proposed Liguasan Marsh Management Commission would oversee the implementation of laws and policies concerning conservation and sustainable use of the marsh’s natural resources.

Despite its protection, the marsh remains under threat from industrial activities such as natural gas extraction and urbanization. A study on the environmental status of the marsh identified high levels of phosphate and mercury in its waters, correlating these pollutants with land-use changes in surrounding municipalities.