Biswajit Das, the Bharatiya Janata Party legislator from Bengal's Bagda assembly constituency, joined Trinamool Congress on Tuesday.
Das is the third BJP leader to switch camp to the ruling Trinamool Congress after the Mamata Banerjee-led party's thumping victory in the assembly polls held earlier this year.
Das's exit from BJP came a day after Bishnupur MLA Tanmoy Ghosh went back to Trinamool Congress, and after political heavyweight Mukul Roy's return in June.
All three of them had ditched the Trinamool Congress for BJP before the assembly elections.