The New Territories is the largest of the three areas of Hong Kong, alongside Hong Kong Island and Kowloon. It makes up 86.2% of Hong Kong's territory, and contains around half of the population of Hong Kong. Historically, it is the region described in the Convention for the Extension of Hong Kong Territory. According to that treaty, the territories comprise the mainland area north of Boundary Street on the Kowloon Peninsula and south of the Sham Chun River (which is the border between Hong Kong and mainland China), as well as over 200 outlying islands, including Lantau Island, Lamma Island, Cheung Chau, and Peng Chau in the territory of Hong Kong.

Later, after New Kowloon was defined from the area between the Boundary Street and the Kowloon Ranges spanned from Lai Chi Kok to Lei Yue Mun, and the extension of the urban areas of Kowloon, New Kowloon was gradually urbanised and absorbed into Kowloon.

The New Territories now comprises only the mainland north of the Kowloon Ranges and south of the Sham Chun River, as well as the Outlying Islands. It comprises an area of 952 square kilometres (368 sq mi). Nevertheless, New Kowloon has remained statutorily part of the New Territories instead of Kowloon.

The New Territories were leased from Qing China by the United Kingdom in 1898 for 99 years in the Second Convention of Peking (The Convention for the Extension of Hong Kong Territory). Upon the expiry of the lease, sovereignty was transferred to the People's Republic of China in 1997, together with the Qing-ceded territories of Hong Kong Island and Kowloon Peninsula.

In 2021, the population of the New Territories was recorded at 3,984,077, with a population density of 4,140 per square kilometre (10,720/sq mi).

Hong Kong Island was ceded to Britain in 1842 by the Treaty of Nanjing ending the First Opium War, and Kowloon south of Boundary Street and Stonecutters Island were ceded in 1860 by a lease subsequently incorporated into the Convention of Beijing ending the Second Opium War. The colony of Hong Kong attracted a large number of Chinese and Westerners to seek their fortune in the city. Its population increased rapidly and the city became overcrowded. The outbreak of bubonic plague in 1894 became a concern to the Hong Kong government. There was a need to expand the colony to accommodate its growing population. The Qing dynasty's defeat in the First Sino-Japanese War had shown that it was incapable of defending itself. Victoria City and Victoria Harbour were vulnerable to any hostile forces launching attacks from the hills of Kowloon.

Alarmed by the encroachment of other European powers in China, Britain also feared for the security of Hong Kong. In June 1898, using the most favoured nation clause that it had previously secured from China, the United Kingdom demanded and received an extension of Kowloon to counter the establishment of France's new Guangzhouwan colony, which was well reinforced by their control of nearby Indochina. In similar fashion, in July, it secured Weihaiwei in Shandong in the north as a base for operations against the Germans in Jiaozhou Bay (Qingdao) and the Russians in Liaodong (Dalian). Chinese officials stayed in the walled cities of Kowloon City and Weihaiwei.

The extension of Kowloon was called the New Territories. The additional land was estimated to be 365 square miles (950 km²) or 12 times the size of the existing colonial Hong Kong at the time.^{[verification needed]}