This article explores data governance design principles for data trustees within data space environments. Data spaces are emerging as novel infrastructures that enable organizations to share data while maintaining data ownership. Accessing data spaces requires significant organizational and technical effort. In our research project, focusing on a geospatial data space, the data trustee plays a central role in enabling access. While this approach appears pragmatic, no comprehensive data governance model for data trustees in data space environments has been developed to date. Building on this need, this article introduces a framework grounded in five core principles: trust, independence, compliance, value-adding services, and stewardship. These principles were derived from a systematic literature review, validated through an expert workshop, and will be applied to a real-world data trustee in a geospatial data space. As the EU continues to promote the development of domain-specific data spaces, this article offers a foundational framework for integrating data trustees into these infrastructures, ultimately enhancing the usability and inclusivity of data sharing ecosystems.