Mediterranean forests are significant yet under-documented carbon reservoirs at the stand scale. This study quantifies the organic carbon stocks of maritime pine (Pinus pinaster Aiton) stands across their three major pools: tree biomass (above- and below-ground), soil organic carbon (0–100 cm) and litter. Twenty-three trees were destructively sampled to measure biomass, moisture and carbon content of the stem, branches, foliage and stump. In parallel, soil carbon, litter, bulk density, pH and moisture were assessed under three treatments (control, open, under-canopy) and four depth classes. Carbon content was determined by loss-on-ignition. Mean total carbon per tree reached 38.2 ± 25.0 kg C (140 kg CO2eq), partitioned among the stem (36.4%), roots (28.6%), foliage (19.9%) and branches (15.1%). Tissue carbon content ranged from 53% to 57% of dry matter, exceeding the IPCC default of 50%. A power allometric model closely related total carbon to diameter (C = 0.454·DBH1.68; R² = 0.89), the best model (DBH²) reaching R² = 0.94. Soil carbon stocks ranged from 100 to 175 t C ha-1 over the 0–100 cm profile and tended to be higher under canopy, with no statistically significant difference among treatments (ANOVA: F = 0.82; p = 0.53). Litter, although modest, was clearly enriched beneath the pine canopy. These results provide allometric and pedological references for estimating carbon stocks of maritime pine plantations in Mediterranean settings.