Manila Bay (Filipino: Look ng Maynila; Spanish: Bahía de Manila) is a natural harbor that serves the Port of Manila (on Luzon), in the Philippines. Strategically located around the capital city of the Philippines, Manila Bay facilitated commerce and trade between the Philippines and its neighboring countries, becoming the gateway for socio-economic development even prior to Spanish occupation. With an area of 1,994 km² (769.9 sq mi), and a coastline of 190 km (118.1 mi), Manila Bay is situated in the western part of Luzon and is bounded by Cavite and Metro Manila on the east, Bulacan and Pampanga on the north, and Bataan on the west and northwest. Manila Bay drains approximately 17,000 km² (6,563.7 sq mi) of watershed area, with the Pampanga River contributing about 49% of the freshwater influx. With an average depth of 17 m (55.8 ft), it is estimated to have a total volume of 28.9×10^⁹ m³ (1,020×10^⁹ cu ft). Entrance to the bay is 19 km (11.8 mi) wide and expands to a width of 48 km (29.8 mi). However, the width of the bay varies from 22 km (13.7 mi) at its mouth and expanding to 60 km (37.3 mi) at its widest point.

The islands of Corregidor and Caballo divide the entrance into two channels, about 2 mi (3.2 km) towards the North and 6.5 mi (10.5 km) wide on the South side. Mariveles, in the province of Bataan, is an anchorage just inside the northern entrance and Sangley Point is the former location of Cavite Naval Base. On either side of the bay are volcanic peaks topped with tropical foliage: 40 km to the north is the Bataan Peninsula and to the south is the province of Cavite.

Across the entrance to Manila Bay are several islands, the largest of which is Corregidor, located 3 kilometers from Bataan and, along with the island of Caballo, separates the mouth of the bay into the North and South Channels. In the south channel is El Fraile Island and outside the entrance, and to the south, is Carabao Island. El Fraile, a rocky island some 4 acres (1.6 ha) in area, supports the massive concrete and steel ruins of Fort Drum, an island fortress constructed by the United States Army to defend the southern entrance of the bay. To the immediate north and south are additional harbors, upon which both local and international ports are situated. Large numbers of ships at the North and South harbors facilitate maritime activities in the bay. Being smaller of the two harbors, the North Harbor is used for inter-island shipping while the South Harbor is used for large ocean-going vessels.

Manila Bay was connected to Laguna de Bay (pronounced "bai") approximately 3,000 years ago. Recurring episodic uplifts along the West Marikina Valley Fault caused the two to break up. Interaction between Manila Bay and Laguna de Bay today occurs only through the Pasig River. The bay was the setting for the Battle of Manila Bay in 1898, in which American troops led by Commodore George Dewey seized the area. This battle showcased the United States' naval strength. All major Spanish ships were destroyed and captured. With its proud historic past and abundant marine life, Manila Bay became the ocean portal and Filipino epicenter for government, economy and industry. During the Russo-Japanese War at the close of the Battle of Tsushima in 1905, three surviving Russian protected cruisers (the Aurora, Zhemchug, and Oleg) managed to make port in then-United States-controlled Manila for repairs. However, because the US was neutral in this conflict, the trio of warships and their crews remained interned by the U.S. until the war officially came to an end in September 1905. During the World War II, Corregidor Island was captured by the Japanese forces based in Manila Bay. Much earlier, various other battles were fought from this naval base, including the Battles of La Naval de Manila in 1646, which finally ended Dutch attempts to seize the Philippines.

The bay remains important for commerce and industry, including fishing, although rapid urban growth and industrialization are contributing to a decline in water quality and deteriorating marine habitats. It also serves a focus for recreation for Metro Manila and is a popular destination for walks and for viewing the sunset. Much of the land fronting the bay along Metro Manila is reclaimed land which now includes important sites such as the Philippine Senate and the Mall of Asia.

On September 27, 2011, the sea walls of Manila Bay were destroyed by the storm surge caused by Typhoon Pedring. Even the United States Embassy, Museo Pambata and Sofitel Philippine Plaza were submerged by the flooding. It was estimated that the damage would cost P30 million to repair. In April 2012, the sea walls were once again opened to the public, having been redesigned to withstand a strong storm surge.

Coastal and marine habitats in the area include upland forests, mangroves, mudflats, sandy beaches, sea grass and coral reefs.

A total of 19,139 birds belonging to 330 families and 99 species were observed at various monitoring sites along the bay area. The endangered Chinese Egret (Egretta eulophotes) and Black-winged cuckoo-shrike were sighted in the area. A large number of migratory birds, catfish and mackerels were once abundant in these waters. Their decline ushered in the appearance of squid, shrimp, and small pelagic species such as herrings and anchovies.