Singh had contested as a PPP candidate from Chamkaur Sahab in 2012 assembly polls, a release issued by the Congress said.
Singh said he had talked to his supporters who wanted him to join the Congress under the leadership of Amarinder.
He said people of Punjab wants to see Amarinder as Chief Minister once again as he had provided a clean and efficient government.
Prominent among those present on the occasion included CLP leader Charanjit Singh Channi, former CLP leader Sunil Jakhar, Kewal Singh Dhillon, Balbir Sidhu, Sukh Sarkaria, Tript Rajinder Singh Bajwa, Navtej Cheema, Gurkirat Singh Kotli and others.