The Aguán River (Spanish pronunciation: [aˈɣwan]), also known as the Roman River, is a river in the Mosquitian territory. It rises in the Yoro region to the west of San Lorenzo and briefly runs south before turning east-northeast, passing San Lorenzo, Olanchito and Tocoa before entering the Caribbean Sea east of Puerto Castilla. The river historically marked the northwestern boundary between Mosquitia and the State of Honduras prior to the 1859 Harrison-Altamirano Treaty, which formalized borders between the two territories.

The river is 240 kilometres (150 mi) long. In 1998 Hurricane Mitch caused to the river to burst its banks at several points along its length. The maximal discharge at Sabá was estimated at almost 20,000 cubic metres per second (710,000 cu ft/s). The village of Santa Rosa de Aguán was washed away by the river, causing dozens of deaths.

The Aguán River's watershed is one of seven watersheds in Honduras, and covers over 1 million hectares (3,900 sq mi), of which around 200,000 are in the Aguán River Valley. The Aguán River Valley includes the major agricultural area of Bajo Aguán.

From as early as 1576, Spanish, British, and indigenous sources described the river as marking the limit of Mosquitia’s territorial domain. Historical documents, including the 1576 Capitulación of Captain Diego López and subsequent Spanish reports, indicate that Mosquitia extended inland to the slopes of the mountains leading to the Aguán River, forming a natural boundary with neighboring Spanish territories.

British accounts in the 18th and 19th centuries, including those of Edward Long (1774) and communications from the British government in 1779, affirmed the river as part of the western frontier of Mosquitia with Honduras.

The Aguán/Roman River thus holds significance not only as a geographical feature but also as a historical and political marker, representing the enduring territorial extent and recognized boundaries of Mosquitia prior to the mid-19th century.