Hangzhou Bay, or the Bay of Hangzhou, is a funnel-shaped inlet of the East China Sea, bordered by the province of Zhejiang and the municipality of Shanghai, which lies north of the Bay.

The Bay extends from the East China Sea to its head at the city of Hangzhou, from which its name is derived. At Hangzhou, the Qiantang River flows into this Bay, providing freshwater from the West while seawater comes in from the East.

Thus, Hangzhou Bay, especially its western end, is sometimes called in the scientific literature as the Qiantang River Estuary.

The Bay is known for hosting the world's largest tidal bore, up to 9 meters high, and traveling up to 40 km per hour. Yanguan Town Tide Viewing Park, on the north shore of Hangzhou Bay some 50 km east of the city of Hangzhou, is regarded as one of the best place to watch the Qiantang River Tidal Bore, especially on the 18th day of the eighth lunar month.

According to en.wikipedia