Motivated by the successful synthesis of penta-silicene nanoribbons using various experimental techniques, we design a new 3D silicon allotrope, labeled cco-Si₄₈, by assembling such nanoribbons, confirm its dynamical, thermal and mechanical stabilities, and further study its electron/phonon transport and linear optical properties based on the state-of-the-art theoretical calculations. We find that cco-Si₄₈ is a direct bandgap semiconductor with a gap of 1.46 eV, exhibiting a high hole mobility in the magnitude of 10³ cm² V⁻¹ s⁻¹ and a low lattice thermal conductivity of 6.33 W m⁻¹ K⁻¹ at 300 K. Unlike the commonly reported n-type silicon-based materials with high thermoelectric performance, the p-type cco-Si₄₈ outperforms its n-type counterpart in the thermoelectric figure of merit (ZT) value with a considerable value of 0.57 at 800 K. We further demonstrate that the electron–phonon interactions play a critical role in determining the optimal carrier concentrations for the peak ZT values. This work expands penta-silicene nanoribbons to their 3D assembled structure with new features and applications.