In this work, we demonstrate that relatively low-scale thermal leptogenesis, accompanied by observable gravitational wave signatures, becomes feasible by extending the type-I seesaw model with a singlet fermion (S) and a singlet scalar ( \(\rho \) ), both of which are odd under a discrete symmetry \(\mathcal {Z}_2\) . At a high scale, the \(\mathcal {Z}_2\) symmetry is spontaneously broken by the vacuum expectation value of \(\rho \) , resulting in: (i) mixing between the right-handed neutrinos ( \(N_2,N_3\) ) and S, and (ii) the formation of domain walls (DWs). In the first scenario, the final lepton asymmetry is produced through the out-of-equilibrium decay of S, which predominantly mixes with \(N_2\) . We demonstrate that the scale of thermal leptogenesis can be lowered to \(M_S\sim 2.8\times 10^6\) GeV, which is three orders of magnitude smaller than the canonical type-I seesaw leptogenesis. In the second scenario, the disappearance of DWs generates observable gravitational wave (GW) signatures, which can be explored through GW experiments.