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To evaluate the nationwide status of grasslands and their soil organic carbon stocks in Japan, we used 1990 as a base year and estimated: (i) the area and the spatial distribution of grasslands, and (ii) the size and the spatial distribution of their soil organic carbon stocks. The carbon stocks were estimated to a 30 cm depth based on datasets containing information on vegetation, wetlands, soils and land classifications. The total grassland area (including natural grasslands, semi-natural grasslands, meadows/pastures and artificial grasslands for non-agricultural use) was estimated at 18 700 km2, which is 5.0% of Japan’s total land area. Nearly two-thirds of the total grassland area contained Andosols or Brown Forest soils, and the percentage of area covered by each soil group differed with grassland type. The estimated soil organic carbon density value (11.4 kg C m–2 for all Japanese grasslands) was consistent with previously reported values (4.0–20.2 kg C m–2 for several Japanese grasslands). A map of the soil organic carbon stocks in grasslands shows a characteristic distribution of high and low carbon stock areas. The total estimated carbon stock in grassland topsoils was 214 Tg C, which is 8.0% of the total carbon stock in Japanese topsoils. The techniques used and the results obtained in this study are valuable for the evaluation of future changes in soil organic carbon stocks in Japan. To obtain more accurate and comprehensive estimates of these changes, it is important to develop the unified grassland categorization system and to collect soil organic carbon data for several types of grasslands, at least meadows/pastures and semi-natural grasslands for agricultural use.