Gdańsk Bay or the Gulf of Gdańsk is a southeastern bay of the Baltic Sea. It is named after the adjacent port city of Gdańsk in Poland.

The western part of Gulf of Gdańsk is formed by the shallow waters of the Bay of Puck. The southeastern part is the Vistula Lagoon, separated by the Vistula Spit and connected to the open sea by the Strait of Baltiysk.

The bay is enclosed by a large curve of the shores of Gdańsk Pomerania in Poland (Cape Rozewie, Hel Peninsula) and the Kaliningrad Oblast of Russia (Sambian Peninsula). The coast of the bay features two very long sandspits, the Hel peninsula and the Vistula Spit. The former defines the Bay of Puck, the latter defines the Vistula Lagoon.

The maximum depth is 120 metres and it has a salinity of 0.7%.

The major ports and coastal cities are Gdańsk, Gdynia, Puck, Sopot, Hel, Kaliningrad, Primorsk and Baltiysk. The main rivers of Gdańsk Bay are the Vistula and the Pregolya. The bay receives the waters of the Vistula direct via three branches—the Leniwka, the Śmiała Wisła and the Martwa Wisła—and indirectly via the Vistula Lagoon with two branches, the Nogat and the Szkarpawa.

By nautic definition, the Gulf of Gdańsk is much larger, including also the area of the Baltic Sea in front of the Russian exclave of Kaliningrad and the Lithuanian coast.

The bayshore was the setting of a naval bombardment during the invasion of Poland, the first combat action of World War II.

The bay plays a pivotal role in the American animated television series Metalocalypse.