Ensuring the timeliness and reliability of master data remains a persistent challenge for many organizations. To mitigate these quality deficits, organizations frequently rely on commercial data brokers. However, this practice creates strategic dependencies and poses significant business risks, particularly as providers typically disclaim liability for the accuracy of the supplied data. In contrast, modern data ecosystems enable the trusted sharing of data assets with strong data sovereignty. In this paper, we address this paradigm shift by deriving a nascent design theory for trustworthy master data management based on self-sovereign identity. The theory is grounded through a hermeneutic literature review combined with industry expert interviews and instantiated through integration into a reference architecture for data spaces. Following an evaluation through additional industry expert interviews, our work provides a framework for a trustworthy master data management in data ecosystems that is reliable, sovereign, and accountable.