Soil organic carbon stock (SOCS) assessments at a national level are essential in the climate change mitigation role. Our study provides the first assessment of the SOC stored in entire Spanish surface soils obtained from one sampling protocol within a regular sampling framework in a short edaphic time. In this study, we analysed topsoil samples (0–30 cm) from 4401 locations. SOC, soil bulk density and stoniness were measured to estimate SOCS. The results showed that half the Spanish areas obtained a SOC value below 1%. There is inherently wide spatial variability in SOC contents in Spainwith lowSOC concentration levels located in southern areas. The lowest SOC levelswere associatedwith agricultural soils. However, no statistically significant differences were found between forestlands and grasslands. The mean SOCS in the Spanish topsoil layer was 56.57 Mg C ha−1 and the total stored in topsoil was 2.8 Pg C. We were surprised to find that our estimates were 40% lower than those known to date for Spain. The results herein highlight the potential land use change for SOC sequestration in Spanish soils. Certain changes in agricultural practises (conservation tillage) or converting some unproductive croplands into grasslands or forestlands can increase carbon sequestration in soils