Due to changes in animal feeding systems in many European countries, a significant portion of grass- lands are subject to degradation due to abandonment or reduced use for feed production, which has resulted in a change in their floristic composition. At the same time, maintaining environmental func- tions of grasslands, particularly soil protection, is also crucial. In this context, a study was conducted on such grasslands to assess soil organic carbon (SOC) in mineral soils (2 sites) and soil organic matter (SOM) in organic soils (2 sites) following overseeding using two types of seed mixtures sown in three different dates: spring, late summer, and autumn. The seed mixtures included grasses and legumes with or without addition of extra herbs species. The experiment comprised also a control treatment without overseeding. Higher SOC in mineral soils and higher SOM in organic soils were recorded in overseeded sites compared to the control. The effect of sowing date was observed only in mineral soils, with the highest SOC content found in sites overseeded in spring. Precise overseeding is recom- mended for degraded grasslands, as it can effectively improve both SOM and SOC content.