Old unmanaged forests are commonly assumed to be carbon neutral; however, there is still a lack of reference studies available to increase the recognition of carbon stock changes in these forests. Studies of old forest carbon storage from hemiboreal regions are very rare compared to temperate and boreal forests in Europe; therefore, the aim of this study was to quantify the carbon stock in hemiboreal over-mature (167–213 years) Norway spruce (Picea abies (L.) Karst.) stands. To explore the total ecosystem carbon pool, the carbon stock of tree biomass, deadwood, and soil in unmanaged (for at least the last 40 years) spruce stands was calculated and compared between different forest site types on dry, wet, and drained mineral soils. Total carbon stock of hemiboreal over-mature spruce stands ranged from 164.8 Mg C ha−1 to 386.7 Mg C ha−1, and 238.5 Mg C ha−1 on average, with no significant differences (p textgreater 0.05) between the forest site types. The carbon stock of tree biomass was significantly affected by the basal area of the upper tree layer (p textless 0.0001) and the interaction between the forest site type and proportion of spruce in the stand composition (p = 0.002). Tree biomass was the dominant carbon pool, followed by soil and deadwood in over-mature spruce stands.