Sona Ram is a former Congress leader who recently quit the party to join BJP.
Singh, 76, had earlier told the party leadership that he wanted to contest from Barmer, the constituency where he was born, else he would contest as an independent candidate.
The party has decided to field ex-Congress man Col (Retd.) Sona Ram Choudhary
However, eventually the party decided to field Sona Ram, who was strongly backed by Rajasthan Chief Minister Vasundhara Raje.
The former army officer has represented Barmer in Parliament earlier, but was defeated in the 2004 election by Singh's son Manavendra Singh, who is now a member of the state legislature. The Congress did not field the Colonel from Barmer in 2009.
According to party sources, Raje told the party leaders that Col Choudhury's caste could be a big draw in Barmer. Sona Ram is a Jat and the caste is the dominant community in Barmer.
Singh, on the other hand, is a Thakur and has never contested from Barmer. She also reportedly hopes that fielding him will help the BJP in getting Jat support in adjoining constituencies.
Singh won Lok Sabha elections from Chittorgarh in Rajasthan in 1991 and 1996, and from Darjeeling in West Bengal in 2009.