The Moga assembly seat fell vacant after Jain submitted his resignation on Wednesday before formally joining the SAD in the presence of Sukhbir. His resignation was accepted by speaker Charanjit Singh Atwal.
Jain had resigned as per the anti-defection law, which guards against any legislator crossing over to another political party.
Sukhbir said Jain's joining the SAD was a shot in the arm for the party.