Whanganui, also spelt Wanganui, is a city in the Manawatū-Whanganui region of New Zealand. The city is located on the west coast of the North Island at the mouth of the Whanganui River, New Zealand's longest navigable waterway. Whanganui is the 19th most-populous urban area in New Zealand and the second-most-populous in Manawatū-Whanganui, with a population of 42,600 as of June 2024.

Whanganui is the ancestral home of Te Āti Haunui-a-Pāpārangi and other Whanganui Māori tribes. The New Zealand Company began to settle the area in 1840, establishing its second settlement after Wellington. In the early years, most European settlers came via Wellington. Whanganui greatly expanded in the 1870s, and freezing works, woollen mills, phosphate works and wool stores were established in the town. Today, much of Whanganui's economy relates directly to the fertile and prosperous farming hinterland.

Like several New Zealand urban areas, it was officially designated a city until an administrative reorganisation in 1989, and is now run by Whanganui District Council. In 2021 it was added to the Design Cities (UNESCO), becoming the only city of design to be located in New Zealand.

The first name of the settlement was Petre (pronounced Peter), named after Lord Petre, an officer of the New Zealand Company, but it was never popular and was officially dropped in 1854. Whanga nui is a Māori language phrase meaning "big bay" or "big harbour".

In the local dialect, Māori pronounce the wh in Whanganui as [ˀw], a voiced labial–velar approximant combined with a glottal stop, but to non-locals the name sounds like "Wanganui" and is hard to reproduce.

In 1991, the New Zealand Geographic Board considered demands from some local Māori to change the name of the river to Whanganui. During a three-month consultation period, the Wanganui District Council was asked for its views and advised the Board that it opposed the change. Letters of both support and opposition were received during this time. After some deliberation, the Board decided to change the spelling of the river's name from "Wanganui" to "Whanganui".

A non-binding referendum was held in Wanganui in 2006, where 82% voted to retain the city's name "Wanganui" without an 'h'. Turnout was 55.4%. Despite the clear results, the spelling of the name continued to be surrounded by significant controversy.

Iwi group Te Rūnanga o Tupoho applied to the New Zealand Geographic Board to change the city's name to "Whanganui" in February 2009, and in late March the Board found there were grounds for the change. The public was given three months to comment on the proposed change, beginning in mid-May. The public submissions were relatively equal, with a slim majority in favour of keeping the status quo. Wanganui Mayor Michael Laws spoke strongly against the proposed change. A second referendum was held in Wanganui in May 2009, and residents again overwhelmingly rejected changing the city's name, with 22% voting to change it to "Whanganui" and 77% voting to retain the name as "Wanganui". Voter turnout was 61%, the highest in a Wanganui referendum, reflecting the widespread controversy. Recognising that the decision was ultimately political in nature, not linguistic, in September 2009 the Geographic Board handed the decision to the Minister for Land Information. Despite the referendum results, the Geographic Board recommended to the Minister that the name should be spelt "Whanganui". In December 2009, the government decided that while either spelling was acceptable, Crown agencies would use the spelling "Whanganui", amending the act to allow other official documents to use "Wanganui", as an alternative official name, if desired.