This bill, titled the Unfunded Mandates Accountability and Transparency Act of 2026, significantly amends the Unfunded Mandates Reform Act of 1995 (UMRA) to enhance transparency and accountability in federal rulemaking. It introduces a new definition for a "major rule," classifying it based on an annual economic impact of $100 million or more, major cost increases, or significant adverse effects on various sectors including competition, employment, and public health. A core provision requires agencies to conduct and publish both initial and final regulatory impact analyses (RIAs) for all proposed and final major rules. These RIAs must quantify anticipated benefits and costs, analyze a reasonable number of regulatory alternatives, assess federal assistance for state/local/Tribal costs, and estimate job impacts. Furthermore, the bill expands stakeholder consultation to include impacted private sector parties, such as small businesses, ensuring early and continuous engagement throughout the rulemaking process. The legislation mandates that agencies select the regulatory alternative that maximizes net benefits for major rules, with specific exceptions requiring approval from the Office of Information and Regulatory Affairs (OIRA). OIRA is also granted new responsibilities for providing guidance and oversight to ensure agency compliance with these principles and to prevent conflicting policies. The bill also extends UMRA's application to independent regulatory agencies, with an exemption for monetary policy, and establishes a process for judicial review to ensure agencies adhere to the new RIA and net benefit requirements.