The laws to secure intellectual property rights on plant germplasm have been strongly developed in parallel to the ongoing seed market consolidation. Germplasm as a commons, i.e., a natural resource accessible to all members of a society, receives almost no legal protection. On the other hand, the use of germplasm and released cultivars in breeding is increasingly restricted by intellectual property rights. In this study, approaches to open source plant germplasm are discussed, and the Open Source Seed (OSS) Licence is introduced and analysed. The OSS Licence was developed by an interdisciplinary working group of plant breeders, agricultural scientists, lawyers, and commons experts in Europe. The aim is to protect germplasm as a commons, support the free exchange of germplasm, stimulate plant breeding, reduce costs, and accelerate innovation. The OSS Licence is a legal tool and novel approach that extends its reach on derivatives of licenced germplasm. It is compatible with current seed laws. Effects on the access to plant germplasm, on breeding for diverse pedoclimatic environments, socioeconomic systems, and on biodiversity as a whole can first show after a few breeding cycles. The impact of open source germplasm on these aspects needs to be monitored carefully.