Although ecosystem management and restoration are known to enhance carbon storage, limited knowledge of ecosystem-specific soil organic carbon (SOC) stocks and processes hinders the development of climate-ready, biodiversity-focused policies. Baseline SOC stocks data for specific ecosystems is essential. This paper aims to: (i) examine SOC stock variability across major grassy ecosystems in Brazil and (ii) discuss data limitations and applications. We compiled the Grassland Synthesis Working Group dataset, which comprehensively aggregates SOC stocks data from published studies on main Brazil's grassy ecosystems. Our dataset results from systematic literature review and regional soil sampling datasets. The dataset provides spatially explicit SOC stocks, physical soil properties, and ancillary information from 182 studies (1996-2021) across 803 sites, spanning 35° latitude and 28° longitude. The dataset, structured in relational tables, reports soil C stocks and ancillary soil parameters at depths up to 100 cm. SOC stocks vary by grassy ecosystem types and sampling depth, with subtropical grasslands (Campos Gerais, South Brazilian highland grasslands, and Pampa) showing the highest SOC stocks across all depth layers (SOC 0-30 cm: 64.5-162.8 Mg C ha