The Pasig River (Filipino: Ilog Pasig; Spanish: Río Pásig) is a water body in the Philippines that connects Laguna de Bay to Manila Bay. Stretching for 25.2 kilometers (15.7 mi), it bisects the Philippine capital of Manila and its surrounding urban area into northern and southern halves. Its major tributaries are the Marikina River and San Juan River. The total drainage basin of the Pasig River, including the basin of Laguna de Bay, covers 4,678 square kilometers (1,806 sq mi).

The Pasig River is technically a tidal estuary, as the flow direction depends upon the water level difference between Manila Bay and Laguna de Bay. During the dry season, the water level in Laguna de Bay is low with the river's flow direction dependent on the tides. During the wet season, when the water level of Laguna de Bay is high, the flow is reversed towards Manila Bay.

The Pasig River used to be an important transport route and source of water for Spanish Manila. Due to negligence and industrial development, the river suffered a rapid decline in the second half of the 20th century and was declared biologically dead in 1990. Two decades after that declaration, however, a renaturation program designed to revive the river has seen the return of life to the river, including eight fish species, 39 species of birds, and 118 species of trees and other vegetation. As a result, the Pasig River received the Asia Riverprize by the International River Foundation in 2019.

The Pasig River Rehabilitation Commission (PRRC) was a Philippine government agency established to oversee rehabilitation efforts for the river from 1999 until it was abolished in November 2019. Rehabilitation efforts are also aided by private sector organizations through raising funds or assisting river cleanups.

The river takes its name from the city of Pasig, which is named after the Tagalog word pasig, meaning "a river that flows into the sea" or "the sandy bank of a river", with the former in reference to the Pasig River's flow from Laguna de Bay towards Manila Bay and out into the South China Sea.

The Pasig River served as an important means of transport; it was Manila's lifeline and center of economic activity. Some of the most prominent kingdoms in early Philippine history, including the kingdoms of Namayan, Maynila, and Tondo grew up along the banks of the river, drawing their life and source of wealth from it. When the Spanish established Manila as the capital of their colonial properties in the Far East, they built the walled city of Intramuros on the southern bank of the Pasig River near its mouth.

After World War II, massive population growth, infrastructure construction, and the dispersal of economic activities to Manila's suburbs left the river neglected. The banks of the river attracted informal settlers and the remaining factories dumped their wastes into the river, making it effectively a huge sewer system. Industrialization had already polluted the river.

In the 1930s, observers noticed the increasing pollution of the river, as fish migration from Laguna de Bay diminished. People ceased using the river's water for laundering in the 1960s, and ferry transport declined. By the 1970s, the river started to emanate offensive smells, and in the 1980s, fishing in the river was prohibited. In 1990, the Pasig River was considered biologically dead by the Danish International Development Agency.