The Fiscal Sponsorship Transparency Act of 2026 amends the Internal Revenue Code to introduce new transparency and accountability measures for charitable organizations involved in fiscal sponsorship arrangements. It requires these organizations to report detailed information about each arrangement, including the names of parties, aggregate amounts transferred or made available, a description of related activities, and the managing officer. This aims to shed light on how funds are managed and utilized in these specific types of partnerships. The bill defines a "fiscal sponsorship arrangement" as one where an applicable organization (excluding private foundations and donor-advised funds) receives and administers funds on behalf of a non-tax-exempt person or solicits funds for a specific project. A crucial element is that the organization must retain discretion and control over the amounts to ensure they further an exempt purpose. Contributions made under an "improper conduit arrangement," where an organization fails to exercise such control, will no longer be considered tax-deductible charitable contributions. To enforce these provisions, the bill establishes new excise taxes on "improper conduit arrangements." An initial tax of 20 percent is imposed on the organization for amounts transferred, along with a 5 percent tax on organization managers who knowingly agree to them. If the improper transfer is not corrected, additional taxes of 100 percent on the organization and 50 percent on non-compliant managers can be levied. The Secretary of the Treasury is directed to issue regulations clarifying these arrangements and the meaning of discretion and control, with the amendments applying to taxable years beginning after December 31, 2027.