Singh (IPA: /ˈsɪŋ/ SING) is a title, middle name, or surname that means "lion" in various South Asian and Southeast Asian communities. Traditionally used by the Hindu Kshatriya community, it was later mandated in the late 17th century by Guru Gobind Singh (born Gobind Das) for all male Sikhs as well, in part as a rejection of caste-based prejudice and to emulate Rajput naming conventions. As a surname or a middle name, it is now found throughout the world across communities and religious groups, becoming more of a generic, caste-neutral, decorative name—similar to names such as Kumar and Lal.

The word Singh is derived from the Sanskrit word सिंह (IAST: siṃha) meaning 'lion', and is used to convey 'hero' or 'eminent person'.

Several variants of the word are found in other languages:

Originally, the Sanskrit word for lion, variously transliterated as Simha or Singh, was used as a title by Kshatriya warriors in northern parts of India. The earliest recorded examples of names ending with "Simha" are the names of the two sons of Rudraraman, who ruled the Western Satraps in the second century CE. Jayasimha, the first ruler of the Chalukya dynasty to bear the title Simha, ruled around 500 CE. The Vengi branch of the Chalukyas continued using Simha as a last name until the eleventh century. The Rajputs started using Singh in preference to the classical epithet of "Varman". Among the Rajputs, the use of the word Singh came into vogue among the Paramaras of Malwa in 10th century CE, among the Guhilots and the Kachwahas of Narwar in the 12th century CE, and the Rathores of Marwar after the 17th century.

By the sixteenth century, "Singh" had become a popular surname among Rajputs. It was adopted by the Sikhs in 1699, as per the instructions of Guru Gobind Singh. Singh is used by all baptized male Sikhs, regardless of their geographical or cultural binding; the women use Kaur. Guru Gobind Singh introduced the surname "Singh" for Sikh men to foster equality and unity within the community. This move aimed to eliminate caste distinctions and promote a fearless and righteous identity. By adopting "Singh," Sikh men were encouraged to live as saint-soldiers, embodying courage, justice, and a commitment to serving humanity, According to Pashaura Singh, the Guru gave male Sikhs the name "Singh", which was associated with aristocracy, to imitate the traditions of the Rajput hill chiefs near Anandpur Sahib.

In the 18th century, several groups started using the title "Singh". These included the Brahmins, the Kayasthas and the Baniyas of what are now Uttar Pradesh and Bihar. In the 19th century, even the Bengal court peons of the lower castes adopted the title "Singh". Bhumihars, who originally used Brahmin surnames, also started affixing Singh to their names. In Bihar and Jharkhand, the surname came to be associated with power and authority, and was adopted by people of multiple castes, including Brahmin zamindars. Citing Kshatriya status, numerous communities use 'Singh' as part of their names. Some Jains have also adopted the surname in addition to various Hindu castes.

Many Muslim Shins historically used the surname "Sing", the earlier form of the name, "Simha", was frequently appended to names found in the Gilgit Manuscripts, a corpus of Buddhist texts and the oldest surviving manuscripts in India, discovered in the Gilgit region of Kashmir.

People belonging to several other castes and communities also started using Singh as a title, middle name or a surname; these include non-Sikh Punjabis, Charans, Yadavs, Gurjars, Brahmins, Marathas, Jats, Kushwahas, Rajpurohits, Kumawats etc. Dalit and other backward groups have also adopted the name, including the Bhils, Koeris, and Dusadhs. The name is also found among the Indian diaspora.