Sikkim (/ˈsɪkɪm/ SIK-im; Nepali: [ˈsikːim]) is a state in northeastern India. It borders Bhutan in the east, the Tibet Autonomous Region of China in the north and northeast, Koshi Province of Nepal in the west, and West Bengal in the south. Sikkim is also close to the Siliguri Corridor, which borders Bangladesh. Sikkim is the least populous and second-smallest among the Indian states. Situated in the Eastern Himalaya, Sikkim is notable for its biodiversity, including alpine and subtropical climates, as well as being a host to Kangchenjunga, the highest peak in India and third-highest on Earth. Sikkim's capital and largest city is Gangtok. Almost 35% of the state is covered by Khangchendzonga National Park – a UNESCO World Heritage Site.

The Kingdom of Sikkim was founded by the Namgyal dynasty in the 17th century. It was ruled by Buddhist priest-kings known as the Chogyal. It became a princely state of the British Indian Empire in 1890. Following Indian independence, Sikkim continued its protectorate status with the Union of India after 1947 and the Republic of India after 1950. It enjoyed the highest literacy rate and per capita income among Himalayan states. In 1973, anti-royalist riots took place in front of the Chogyal's palace. In 1975, after the Indian Army took over the city of Gangtok, a referendum was held that led to the dissolution of the monarchy and Sikkim's joining India as its 22nd state.

Modern Sikkim is a multiethnic and multilingual Indian state. The predominant religion is Hinduism, with a significant Vajrayana Buddhist minority. Sikkim's economy is largely dependent on agriculture and tourism. As of 2019^[update], the state had the fifth-smallest GDP among Indian states, although it is also among the fastest-growing.

The name Sikkim is believed to be a combination of the Limbu words su "new" and khyim "palace" or "house". The Tibetan name for Sikkim is Drenjong (འབྲས་ལྗོངས, Wylie transliteration: 'bras ljongs), which means "valley of rice", while the Bhutias call it Beyul Demazong, which means "the hidden valley of rice". According to folklore, after establishing Rabdentse as his new capital, Bhutia king Tensung Namgyal built a palace and asked his Limbu Queen to name it. The Lepcha people, the original inhabitants of Sikkim, called it Nye-mae-el, meaning "paradise". In historical Indian literature, Sikkim is known as Indrakil, the garden of the war god Indra.

The Lepchas are considered to be the earliest inhabitants of Sikkim. However the Limbus and the Magars also lived in the inaccessible parts of West and South districts as early as the Lepchas perhaps lived in the East and North districts. The Buddhist saint Padmasambhava, also known as Guru Rinpoche, passed through the land in the 8th century. The Guru is reported to have blessed the land, introduced Buddhism, and foretold the era of monarchy that would arrive in Sikkim centuries later.^{[citation needed]}

According to legend, Khye Bumsa, a 14th-century prince from the Minyak House in Kham of Tibetan ancestry, received a divine revelation instructing him to travel south to seek his fortunes. A fifth-generation descendant of Khye Bumsa, Phuntsog Namgyal, became the founder of Sikkim's monarchy in 1642, when he was consecrated as the first Chogyal, or priest-king, of Sikkim by the three venerated lamas at Yuksom. Phuntsog Namgyal was succeeded in 1670 by his son, Tensung Namgyal, who moved the capital from Yuksom to Rabdentse (near modern Pelling). In 1700, Sikkim was invaded by the Bhutanese with the help of the half-sister of the Chogyal, who had been denied the throne. The Bhutanese were driven away by the Tibetans, who restored the throne to the Chogyal ten years later. Between 1717 and 1733, the kingdom faced many raids by the Nepalese in the west and Bhutanese in the east, culminating with the destruction of the capital Rabdentse by the Nepalese. In 1791, the Dalai Lama sent troops to support Sikkim and defend Tibet against the Gorkha Kingdom. Following the subsequent defeat of Gorkha, an alliance of Sino-Tibetans established control over Sikkim.

Following the beginning of British rule of neighbouring India, Sikkim allied with Britain against their common adversary, Nepal. The Nepalese attacked Sikkim, overrunning most of the region including the Terai. This prompted the British East India Company to attack Nepal, resulting in the Gurkha War of 1814. Treaties signed between Sikkim and Nepal resulted in the return of the territory annexed by the Nepalese in 1817. However, ties between Sikkim and the British weakened when the latter began taxation of the Morang region. In 1849, two British physicians, Sir Joseph Dalton Hooker and Archibald Campbell, the latter being the superintendent of Darjeeling, ventured into the mountains of Sikkim with the prior permission of King Tsugphu Namgyal. The doctors were detained by an influential Dewan, leading to a punitive British expedition against the kingdom, after which the Darjeeling district and Morang were annexed to British India in 1853. The Chogyal of Sikkim became a titular ruler under the directive of the British governor as a result of the invasion.

Sikkim became a British protectorate in the later decades of the 19th century, formalised by a convention signed with China in 1890. Sikkim was gradually granted more sovereignty over the next three decades, and became a member of the Chamber of Princes, the assembly representing the rulers of the Indian princely states, in 1922.