Thunder Bay is a large bay on the northern shore of Lake Superior, in Thunder Bay District, Ontario, Canada. The bay is about 50 kilometres (31 mi) long and 22 kilometres (14 mi) wide. It is bordered to the east by the Sibley Peninsula at the southern tip of which is Thunder Cape, marking the entrance to the bay for ships approaching from the east. The mesas and sills on the peninsula are known as the Sleeping Giant due to their appearance when viewed from Thunder Bay.

Notable islands and island chains in the bay include:

• Pie Island and nearby Flatland Island
• Welcome Islands
• Caribou Island

Rivers emptying into the bay include the:

• Kaministiquia River
• Neebing River
• McIntyre River
• Current River
• MacKenzie River (18 km east of the city)
• Blende River
• Wild Goose Creek
• Blind Creek

The harbour at the City of Thunder Bay is Canada's westernmost port on the Great Lakes, and the end of Great Lakes navigation.

The Ojibwa called it Animikie, meaning "thunder". French explorers called it Baie du Tonnerre which was translated to Thunder Bay in English. In 1871, the bay gave its name to the newly created Thunder Bay District, and in 1970, the amalgamated city of Port Arthur and Fort William also adopted the name Thunder Bay.

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48°28′N 89°00′W / 48.467°N 89.000°W / 48.467; -89.000