Leader Mahantha Thakur has filed his candidacy from Madhesh for the National Assembly election.
On Wednesday, Thakur registered his nomination on behalf of the Janata Samajwadi Party (JSP) Nepal.
The Nepali Congress, CPN-UML, and JSP Nepal earlier agreed a seat-sharing deal to contest the National Assembly election.
According to leaders, an informal understanding has already been reached to split the 18 National Assembly seats among the three parties, with the Nepali Congress taking nine seats, the CPN-UML eight seats, and the JSP Nepal one seat.
Thakur filed his nomination at the Election Office in Janakpur on Wednesday. He previously won direct elections to the House of Representatives twice from Mahottari Constituency No. 3.
The Loktantrik Samajwadi Party (LSP) and the JSP Nepal decided to unify on December 29. Following the unification, the JSP Nepal named Thakur as its candidate for the National Assembly. Before merging with the JSP Nepal, Thakur was the chairman of the LSP.