Abstract. Forests play an important role in the global carbon (C) cycle, and the South Korean forests also contribute to this global C cycle. While the South Korean forest ecosystem was almost completely destroyed by exploitation and the Korean War, it has successfully recovered because of national-scale reforestation programs since 1973. There have been several studies on the estimation of C stocks and balances over the past decades in the South Korean forests. However, a retrospective long-term study that includes biomass and dead organic matter C and validates dead organic matter C is still lacking. Accordingly, we estimated the C stocks and their changes of both biomass and dead organic matter C during the 1954–2012 period using a process-based model, the Korean Forest Soil Carbon model, and the 5th South Korean national forest inventory (NFI) report. Validation processes were also conducted based on the 5th NFI and statistical data. Simulation results showed that the biomass C stocks increased from 36.4 to 440.4 Tg C at a rate of 7.0 Tg C yr⁻¹ during the period 1954–2012. The dead organic matter C stocks increased from 386.0 to 463.1 Tg C at a rate of 1.3 Tg C yr⁻¹ during the same period. The estimates of biomass and dead organic matter C stocks agreed well with observed C stock data. The annual net biome production (NBP) during the period 1954–2012 was 141.3 g C m⁻² yr⁻¹, which increased from −8.8 g C m⁻² yr⁻¹ in 1955 to 436.6 g C m⁻² yr⁻¹ in 2012. Because of the small forested area, the South Korean forests had a comparatively lower contribution to the annual C sequestration by global forests. In contrast, because of the extensive reforestation programs, the NBP of South Korean forests was much higher than those of other countries. Our results could provide the forest C dynamics in South Korean forests before and after the onset of reforestation programs.