Waitangi Day (Māori: Te Rā o Waitangi), the national day of New Zealand, marks the anniversary of the initial signing—on 6 February 1840—of the Treaty of Waitangi. The Treaty of Waitangi was an agreement towards British sovereignty by representatives of the Crown and indigenous Māori chiefs, and so is regarded by many as the founding document of the nation.

Waitangi Day was first celebrated in 1934, and it was made a national public holiday in 1974. The anniversary is marked annually on 6 February, and a public holiday is observed on the day, or if the date falls on a Saturday or Sunday then the following Monday is observed.

Ceremonies take place at Waitangi and elsewhere to commemorate the signing of the treaty. A variety of events are held, including parties, Māori hui (social gatherings), reflections on New Zealand history, official awards and citizenship ceremonies. The commemoration has also been the focus of protest by Māori activists, and is occasionally the focus of controversy.

The Treaty of Waitangi (Māori: Te Tiriti o Waitangi) was first signed on 6 February 1840 on the grounds of James Busby's house—now known as Treaty House—at Waitangi, in the Bay of Islands. The treaty was signed by representatives acting on behalf of the British Crown and, initially, by about 45 Māori chiefs. Over the course of the next seven months, copies of the treaty were toured around the New Zealand archipelago by the British, and eventually around 540 Māori chiefs would sign. The signing had the effect of securing British sovereignty over the islands of New Zealand, which was officially proclaimed by the Lieutenant-Governor of New Zealand, William Hobson, on 21 May 1840.

Prior to 1934, most celebrations of New Zealand's founding as a colony were held on 29 January, the date on which William Hobson arrived in the Bay of Islands to issue the proclamation of his appointment, which had been prepared by colonial office officials in England. Hobson had no draft treaty. From the British perspective the proclamation was the key legal document, "what the treaty said was less important".

In 1932, Governor-General Lord Bledisloe and his wife purchased and presented to the nation the run-down house of James Busby, where the treaty was initially signed. They subsequently donated £500 to restore the building. The Treaty House and grounds were made a public reserve, which was dedicated on 6 February 1934. This event is considered to be the first Waitangi Day.

In 1940, another event was held at the grounds, commemorating the 100th anniversary of the treaty signing. The event was a success and helped raise the profile of the treaty and its day of observance in the national consciousness.

Annual commemorations of the treaty signing began in 1947. The 1947 event was a Royal New Zealand Navy ceremony centring on a flagpole which the Navy had paid to erect in the grounds. The ceremony was brief and featured no Māori. The following year, a Māori speaker was added to the line-up, and subsequent additions to the ceremony were made nearly every year. From 1952, the governor-general attended, and from 1958 the prime minister also attended, although not every year. From the mid-1950s, a Māori cultural performance was usually given as part of the ceremony. Many of these early features remain a part of Waitangi Day ceremonies, including a naval salute, the Māori cultural performance (now usually a pōwhiri, a ceremonial welcome), and speeches from a range of Māori and Pākehā (non-Māori) dignitaries.