The by-poll was necessitated after Chief Minister Pawan Chamling resigned from the seat and retained Namchi-Singhithang after the April assembly polls.
While the Sikkim Democratic Front (SDF) contestant is a 32-year-old woman, Kumari Manger, the Bharatiya Janata Party has nominated 26-year-old Bikash Basnet.
Basnet had contested in the April polls on a Sikkim Krantikari Morcha (SKM) ticket but lost by over 3,000 votes.
The SKM is not in the fray this time, and is lending support to the BJP.
Bishnu Prasad Adhikari of the Congress and Rup Narayan Chamling, chief minister Chamling's brother and an independent candidate are also contesting.
There are over 11,700 electors, who will vote in 14 polling stations in three sectors.