For the first time, the thermoelectric properties of cement/single-walled carbon nanotube (SWCNT) nanocomposites with over 3, 7, 14 and 28 days of hydration are reported, while a thermoelectric generator device (TEG) is fabricated utilising the material with the highest achieved power factor (PF). SWCNTs with inherent p-type semiconductor characteristics were introduced into the cementitious matrix at a water to cement (w/c) ratio of 0.5, while the Seebeck coefficient (S), electrical conductivity (σ) and PF for different SWCNT loadings were determined at different ages. The 28 day cement/SWCNT sample with 0.5 wt% loading exhibited the highest performance in terms ofσ(1.59 × 10³S m⁻¹),S(+ 1348.8 μV K⁻¹) and PF (2.89 × 10³μW m⁻¹K⁻²), with the PF being the highest among other carbon nanoadditive-based cement thermoelectric nanocomposites to date. The cement/SWCNT(0.5) was used for the fabrication of a thermoelectric generator (TEG) device exhibiting a maximum power output (P_max) of 5.02 μW and a power density of 5.02 mW m⁻², upon being exposed to a temperature difference (ΔT) of 50 K. Finite element (FE) simulations validated the TEG thermoelectric performance corroborating the experimental findings. The highly efficient cement/SWCNT thermoelectric nanocomposites, as well as the cement-based structural TEG device, demonstrated herein could open new avenues towards future energy efficient buildings and green construction.