Moisture at the steel-concrete interface (SCI) can influence the durability of reinforced concrete but a fundamental understanding of its effects on corrosion initiation and propagation remains inconclusive. This is partly due to a lack of reliable, non-destructive monitoring techniques. This study introduces a novel approach leveraging 3D-printed reinforcement spacers with embedded RFID-MEMS sensors to enable in situ, real-time relative humidity monitoring at the SCI. Concrete specimens with different water-to-cement ratios (0.40, 0.55) and curing ages (3, 14 days) were exposed to 50 °C and 65 % RH, 21 °C to produce varying degrees of porosity and moisture content. The results show that precise measurements with a maximum difference of ± 2 % can be obtained for exposure humidity up to 90 % RH. However, a reduction in precision occurred above 93 % RH due to condensation effects. Crucially, this work demonstrates the first successful integration of RFID-MEMS sensors in 3D-printed spacers to quantify SCI moisture states non-destructively, providing actionable insights for corrosion risk assessment tailored to concrete composition and environmental exposure.