The "Clean Shipping Act of 2025" amends the Clean Air Act to introduce significant new environmental regulations for the shipping industry. Its primary goal is to establish a Marine Greenhouse Gas Fuel Standard , requiring the Environmental Protection Agency (EPA) to set progressively stringent limits on the carbon intensity of fuel used by large commercial vessels on voyages to or between U.S. ports. Under this standard, vessels of 400 gross tonnage or more must reduce their fuel's carbon intensity by at least 30 percent below a 2027 baseline by 2030, escalating to a 100 percent reduction by 2050. The EPA is tasked with promulgating these standards, with provisions for adjusting targets if they are not technologically or economically feasible, while considering overall emission reductions. The bill also allows for harmonization with international standards and provides exemptions for vessels with limited U.S. port calls. Furthermore, the legislation mandates comprehensive monitoring and reporting requirements, obligating vessel owners to submit annual data on fuel carbon intensity and greenhouse gas emissions for covered voyages, which the EPA will compile and publish. A second key provision directs the EPA to establish in-port marine vessel zero emission standards by January 1, 2029, aiming to eliminate all greenhouse gas and air pollutant emissions from vessels at anchorage or berth in the U.S. contiguous zone by January 1, 2035. These in-port emission targets can also be adjusted based on technological and economic feasibility.