Singh, who was elected to Lok Sabha from Mandi Parliamentary Constituency in May 2009, won the Assembly election and became the chief minister for the sixth time.
Singh, who is in Delhi, has sent his resignation to the Speaker of Lok Sabha Meira Kumar, party sources said here.
Singh, along with Irrigation and Public Health minister Vidya Stokes, would be taking oath as MLA for eighth time on first day of the Assembly session on January 8.
With the resignation of the 78-year-old leader from Lok Sabha, Mandi seat would be vacated and by-election for this seat would be held soon.
