  Right-handed neutrinos ($\nu_{R}$) offer an intriguing portal to new physics
in hidden sectors where dark matter (DM) may reside. In this work, we delve
into the simplest hidden sector involving only a real scalar exclusively
coupled to $\nu_{R}$, referred to as the $\nu_{R}$-philic scalar. We
investigate the viability of the $\nu_{R}$-philic scalar to serve as a DM
candidate, under the constraint that the coupling of $\nu_{R}$ to the standard
model is determined by the seesaw relation and is responsible for the observed
DM abundance. By analyzing the DM decay channels and solving Boltzmann
equations, we identify the viable parameter space. In particular, our study
reveals a lower bound ($2.6\times10^{5}$ GeV) on the mass of $\nu_{R}$ for the
$\nu_{R}$-philic scalar to be DM. The DM mass may vary from sub-MeV to sub-GeV.
Within the viable parameter space, monochromatic neutrino lines from DM decay
can be an important signal for DM indirect detection.
