In this study, low-background γ-spectrometry was used to determine the 228 Ra/ 226 Ra ratio of 131 coastal water samples from various environments around Honshu Island, Japan (mainly around Noto Peninsula) at 1–3 month intervals from April 2003 until September 2005. Spatial variation in 228 Ra/ 226 Ra ratios was also assessed by analyzing 34 coastal water samples from five areas within the Sea of Japan during May and June 2004.The 228 Ra/ 226 Ra ratio of coastal water from all sites around Noto Peninsula shows seasonal variation, with minimum values during summer ( 228 Ra/ 226 Ra=0.7) and maximum values during autumn–winter ( 228 Ra/ 226 Ra=1.7–2). This seasonal variation is similar to that recorded for coastal water between Tsushima Strait and Noto Peninsula. The measured lateral variation in 228 Ra/ 226 Ra ratios within coastal water between Tsushima Strait and Noto Peninsula is only minor (0.5–0.7; May–June 2004). Coastal waters from two other sites (Pacific shore and Tsugaru Strait, north Honshu) show no clear seasonal variation in 228 Ra/ 226 Ra ratio.These measured variations in 228 Ra/ 226 Ra ratio, especially the temporal variations, have important implications for seasonal changes in patterns of coastal water circulation within the Sea of Japan.