New Brunswick is bounded on the north by Quebec’s Gaspé Peninsula and by Chaleur Bay. The eastern boundary is formed by the Gulf of Saint Lawrence and Northumberland Strait. The southeast corner of the province is connected to the Nova Scotia peninsula by the narrow Isthmus of Chignecto. The south of the province is bounded by the Bay of Fundy coast, (which with a rise of 16 m (52 ft), has amongst the highest tides in the world). The US state of Maine forms the western boundary.

New Brunswick differs from the other Maritime provinces physiographically, climatologically, and ethnoculturally. Both Nova Scotia and Prince Edward Island are either surrounded by, or are almost completely surrounded by water. Oceanic effects therefore tend to define their climate, economy, and culture. On the other hand, New Brunswick, although having a significant seacoast, is sheltered from the Atlantic Ocean proper and has a large interior that is removed from oceanic influences. As a result, the climate tends to be more continental in character rather than maritime. The major river systems of the province include the St. Croix River, Saint John River, Kennebecasis River, Petitcodiac River, Magaguadavic River, Miramichi River, Nepisiguit River, and the Restigouche River. Although smaller, the Bouctouche River, Richibucto River and Kouchibouguac River are also important.

The settlement patterns and the economy of New Brunswick are based more on the province’s river systems than its seacoasts. Northern New Brunswick is dominated by the Appalachian Mountains within the Eastern Canadian forests ecoregion, with the northwestern part of the province consisting of the remote and rugged Miramichi Highlands as well as the Chaleur Uplands and the Notre Dame Mountains, with a maximum elevation at Mount Carleton of 817 m (2,680 ft). The New Brunswick Lowlands form the eastern and central portions of the province and are part of the Gulf of St. Lawrence lowland forests ecoregion. Finally the Caledonia Highlands and St. Croix Highlands extend along the Bay of Fundy coast reaching elevations of more than 400 m (1,312 ft).

The total land and water area of the province is 72,908 km2 (28,150 sq mi), over 80 percent of which is forested. Agricultural lands are found mostly in the upper St. John River Valley, with lesser amounts of farmland in the southeast of the province, especially in the Kennebecasis and Petitcodiac river valleys. The three major urban centres are all in the southern third of the province.