Wanganui is located close to the mouth of the Whanganui River on the west coast of New Zealand's North Island. It isapproximately 200 kilometres north of Wellington and 75 kilometres northwest of Palmerston North, at the junction of State Highways 3 and 4. Most of the town lies on the river's northwestern bank, although some suburbs are located on the opposite side of the river.

It enjoys a temperate climate, with slightly above the national average sunshine (2100 hours per annum), and about 900 mm of annual rainfall. Frosts in winter are uncommon, but not rare.