| Honorific-Prefix: | General Kaji |
| Nain Singh Thapa | |
| Native Name: | जनरल काजी नैनसिंह थापा |
| Birth Date: | c. AD 1777 |
| Birth Place: | Borlang, Gorkha district |
| Death Date: | 1728 Saka Era (1806/07 AD) |
| Death Place: | Kangra Fort, Garhwal Kingdom |
| Father: | Amar Singh Thapa (sardar) |
| Mother: | Satyarupa Maya |
| Children: | Mathabar Singh Thapa, Queen Tripurasundari of Nepal, Ujir Singh Thapa, Ganesh Kumari (mother of Jung Bahadur Rana) |
| Residence: | Thapathali Durbar |
| Rank: | General |
| Battles: | Battles of Unification of Nepal |
| Allegiance: |  Nepal |
| Relatives: | Bhimsen Thapa (brother) Ranajit Pande (father-in-law) Balbhadra Kunwar (nephew) Jang Bahadur Rana (grandson) |
Nain Singh Thapa or Nayan Singh Thapa (Nepali: नैनसिंह थापा/नयनसिंह थापा) (died late 1806 or early 1807) was a Nepalese Kaji (minister) and a military general. He died in the offensive campaign of Kangra from bullet injury. He was the owner of the Thapathali Durbar temple complex.
He was a Kaji and General of Nepal Army. A royal order was decreed on Ashwin Sudi 2, 1862 V.S. (September 1805), for the preparations of the Kangra campaign. In September 1805, while being deputed at Kangra Fort, his brother Mukhtiyar Bhimsen Thapa ordered him to arrest military deserters. Gorkhali forces under Badakaji Amar Singh Thapa, Rudrabir [Shah] and Nain Singh overran Nalagarh and crossed Sutlej river. They fought against and defeated King Sansar Chand at Mahal Mori in May 1806. Sansar Chand fled to Kangra fort after taking refuge at Sujanpur Tira. Widow of Kirti Chand, Commander of Kangra Army and Nain Singh, the Nepalese commander led the battle at Tira Sujanpur. The Gorkhali invasion became persistent and irresistible. On Saturday V.S. 1863 Kartik Badi 13 (i.e. 8 November 1806), there was a letter which positioned Bhakti Thapa under the joint authority of Badakaji Amar Singh Thapa and Nain Singh. Nain Singh came with a reinforcement of 1500 men along with Sardar Udatta Shahi leading 3 companies while Subba Ranganath Gurung and Prahlad Gurung had led 4 companies. Nain Singh and Amar Singh were entrusted with the main Nain Singh fought at Kangra fort and was mortally wounded from which he died in the winter of 1806/1807. The Bhasavamshawali also states the death of Nain Singh on 1728 Saka Era i.e. (1806/7) A.D. The event was sketched by 19th-century Garhwali poet and painter Mola Ram. In an 1852 interview, Jang Bahadur Rana mentioned the death of his maternal grandfather Nain Singh at Kangra.
See main article: Thapa dynasty. He was born as second son to Sanukaji Amar Singh Thapa. Nayan Singh Thapa had 4 brothers – Bhimsen Thapa, Bhaktawar Singh, Amrit Singh and Ranbir Singh and two step-brothers – Ranzawar and Ranbam. He was father of Mathabarsingh Thapa, Queen Tripurasundari of Nepal and grandfather of Jung Bahadur Rana. He was the son-in-law of Chief Kazi Ranajit Pande of noble Pande family and father-in-law of Kazi Bal Narsingh Kunwar of the noble Kunwar Rana family.[1] Kumar Pradhan asserts that Sher Jung Thapa was son of Nain Singh Thapa, whom Bhimsen adopted while Baburam Acharya contradicts that Sher Jung Thapa was nephew of Mathabarsingh Thapa and was sixteen years old on April 1835.
He was the owner of Thapathali Durbar.