Abstract: A large amount of artificial radionuclides have been released into the ocean, contributing to serious nuclear pollution in marine environment, and arising public concerns and worry around the world. The Fukushima-derived artificial radionuclides can also be used as tracers to reveal the migration, transformation processes, and fate of artificial radionuclides in the ocean. The most polluted port within less than 1 km from the Fukushima Daiichi Nuclear Power Plant (FDNPP) was focused on in this study. The most polluted port near the FDNPP serves as windows to reflect progresses and effectiveness of decommissioning of the FDNPP, which is inaccessible for public and many other counties around the world. Historical activities of ^134,137Cs in seawater, marine sediment, and marine fish were reconstructed from April 2011 to October 2023 on the basis of over 1 000 reports from Ministry of Economy, Trade and Industry of Japan, Nuclear Regulation Authority of Japan, and Tokyo Electric Power Company. The patterns of the three-stage evolution of ^134,137Cs in seawater, the four-stage evolution of ^134,137Cs in sediments, and the three-stage evolution of ^134,137Cs in marine fish were proposed to quantify the activity levels and effective half-lives (EHL) of ^134,137Cs at different stages. The evolutions of historical ^134,137Cs in seawater, sediment, and marine fish were closely related to multiple countermeasures of decommissioning at the FDNPP, including the relocation of the drainage channels during June 2014 to April 2015, seabed covering of port in April 2015, removal of highly contaminated retained water in December 2015, filling of tunnels and towers in December 2015, and completed construction of sea-side impermeable walls in February 2016. The longest EHL of ^134,137Cs in marine sediment indicates the memory effect of marine sediment and its persistent and dominated contribution to ^134,137Cs in marine fish. Additionally, a highly consistent activity ratio of ¹³⁴Cs to ¹³⁷Cs (about 1.0) was simultaneously calculated in seawater, sediment, and marine fish, indicating the transferring of the Fukushima-derived ^134,137Cs in multiple matrices in the marine environment. The temporal variation of concentration factor of ¹³⁷Cs in marine fish was also constructed to reveal the dynamic processes of the enrichment and uptake of ¹³⁷Cs in marine fish from seawater. The relatively high value of concentration factor of ¹³⁷Cs in marine fish was observed during the initial period of nuclear accident followed by a decline in concentration factor of ¹³⁷Cs to about 100 L/kg. This study would provide scientific evaluations for the effectiveness of the decommissioning of the FDNPP and the consequences of Fukushima contaminated water discharged into the ocean.