The United Nations Convention on Transparency in Treaty-based Investor-State Arbitration entered into force for the European Union on March 25, 2026, six months after the Union approved the Convention on September 25, 2025 and became its tenth party. The Mauritius Convention extends the UNCITRAL Rules on Transparency, covering public access to documents, open hearings, and third-party submissions, to investor-state arbitrations under investment treaties concluded before April 1, 2014, which would otherwise remain governed by older confidentiality defaults. Relatedly, when signing the Convention in 2024, the EU has also committed to fund the UNCITRAL Transparency Registry for a further three years. The Union’s accession materially expands the Convention’s reach and creates a template for its member states, most of which have yet to ratify individually. Transparency remains the least contested corner of ISDS reform, and every extension of the Rules to the pre-2014 treaty stock, where most claims still arise, narrows the space for arbitrations conducted wholly out of public view.