Rajendra Gavit, the Bharatiya Janata Party's sitting Member of Parliament from Palghar Lok Sabha seat, joined the Shiv Sena on Tuesday.
He was welcomed in the party by Shiv Sena chief Uddhav Thackeray.
Gavit will be Shiv Sena's candidate from Palghar.
Gavit had joined the BJP in May 2018 from the Congress ahead of the by-polls, which he won defeating Shiv Sena by 29,572 votes.
The seat was previously held by sitting BJP MP Chintaman Wanaga. The election was necessitated after his death.