The Fukushima Dai-ichi Nuclear Power Plants (FDNPPs) accident in 2011 led to an unprecedented release of radionuclides into the environment. Particularly important are 90 Sr and 137 Cs due to their known health detriments and long half-lives (T 1/2 ≈ 30 y) relative to ecological systems. These radionuclides can be combined with the longer-lived 129 I (T 1/2 = 15.7 My) to trace hydrologic, atmospheric, oceanic, and geochemical processes. This study seeks to evaluate 137 Cs, 90 Sr, and 129 I concentrations in seawater off the coast of Japan, reconcile the sources of contaminated waters, and assess the application of 137 Cs/ 90 Sr, 129 I/ 137 Cs, and 129 I/ 90 Sr as oceanic tracers. We present new data from October 2015 and November 2016 off the coast of Japan, with observed concentrations reaching up to 198 ± 4 Bq•m −3 for 137 Cs, 9.1 ± 0.7 Bq•m −3 for 90 Sr, and (114 ± 2) × 10 −5 Bq•m −3 for 129 I. The utilization of activity ratios suggests a variety of sources, including sporadic and independent releases of radiocontaminants. Though overall concentrations are decreasing, concentrations are still elevated compared to pre-accident levels. In addition, Japan's Environment Minister has suggested that stored water from the FDNPPs may be released into the environment and thus continued efforts to understand the fate and distribution of these radionuclides is warranted.