Rajnagar Assembly constituency is an assembly constituency in Madhubani district in the Indian state of Bihar.

As per Delimitation of Parliamentary and Assembly constituencies Order, 2008, No.

[1] Rajnagar Assembly constituency is part of No.

7 Jhanjharpur (Lok Sabha constituency).

[1] In the Bihar Legislative Assembly election of 2015, the Communist Party of India (CPI) won Rajnagar seat by defeating rival Ramavtar Paswan of RJD.