Monitoring of 137Cs in seawater in coastal areas around Japan between 1983 and 2016 yielded new insights into the sources and transport of Fukushima Daiichi Nuclear Power Plant (FDNPP)-derived 137Cs, particularly along the west coast of Japan. Before the FDNPP accident (1983-2010), the activity concentrations of 137Cs, mainly from fallout, were decreasing exponentially. Effective 137Cs half-lives in surface seawater ranged from 15.6 to 18.4 yr. After the FDNPP accident (March 2011) 137Cs activity concentrations in seawater off Fukushima and neighboring prefectures immediately increased. Since May/June 2011, 137Cs activity concentrations there have been declining, and they are now approaching preaccident levels. Along the west coast of Japan remote from FDNPP (i.e., the Japan Sea), however, radiocesium activity concentrations started increasing by 2013, with earlier (May/June 2011) increases at some sites due to airborne transport and fallout. The inventory of 137Cs in the Japan Sea (in the main body of the Tsushima Warm Current) in 2016 was calculated to be 0.97 × 1014 Bq, meaning that 0.44 × 1014 Bq of FDNPP-derived 137Cs was added to the estimated global fallout 137Cs inventory in 2016 (0.53 × 1014 Bq). The net increase of 137Cs inventory in the Japan Sea through the addition of FDNPP-derived 137Cs accounts for approximately 0.2% of the total 137Cs flux from the plant to the ocean from the accident.