<p>Arid and semi-arid ecosystems constitute important carbon reservoirs, yet their contribution to climate regulation remains insufficiently documented, particularly in Small Island Developing States (SIDS). In this context, invasive or introduced woody species may contribute substantially to biomass accumulation in degraded drylands, although quantitative evidence for Neltuma juliflora remains limited. This study provides the first block-scale assessment of carbon stocks in N. juliflora stands on Santiago Island, Cabo Verde, using validated allometric equations and topsoil analyses across 11 sampling blocks. A total of 463 individuals were measured. Aboveground carbon stocks varied markedly among blocks, ranging from 27.4 to 85.3 Mg C ha⁻¹, and were strongly associated with variation in stand structure, particularly equivalent diameter, tree height and stand density. In contrast, soil organic carbon in the 0–20 cm layer remained comparatively low, averaging 7.1 Mg C ha⁻¹, and showed no significant radial trend with increasing distance from the tree. Instead, variation in soil carbon was more closely associated with block-level edaphic heterogeneity than with local canopy effects. Belowground carbon, estimated using a fixed root-to-shoot ratio, followed the same spatial pattern as aboveground biomass and should therefore be interpreted as a derived rather than independently measured pool. Overall, the results demonstrate that N. juliflora can support substantial woody carbon storage under semi-arid insular conditions, whereas shallow soil carbon remains limited. These findings provide an important baseline for dryland carbon assessment in Cabo Verde and support a cautious, site-specific evaluation of the species in restoration and climate-mitigation strategies given its recognised invasive potential.</p>