The 80-year-old former Union Home Minister was welcomed into the party fold by senior leaders like Ajay Maken and PC Chacko.
Singh had contested the last two Lok Sabha elections as an independent candidate from Jalore in Rajasthan, a constituency he had represented earlier in the Parliament.
Singh also heads Safai Mazdoor Congress, a major organization working for the welfare of sanitation workers.
He was the Home Minister between 1986 and 1989 in the Rajiv Gandhi Cabinet.
He had also served as Agriculture Minister in Gandhi Cabinet during 1984-1986.