Mountain–plain transitional landscapes are especially important as groundwater recharge zones. In this study, the oxygen and hydrogen stable isotopic composition (δ¹⁸O and δD) of water and hydrochemical information were employed to quantify contribution ratios of different sources of groundwater recharge in the Ashikaga area of central Japan. The study area is situated between the Ashio Mountains and the Kanto Plain, and the Watarase River flows into the region parallel to the mountain–plain boundary. There was an obvious isotopic altitude effect in and around the study area (−0.25‰ per 100 m for δ¹⁸O and −1.7‰ per 100 m for δD), and the isotopic signatures of water from the Watarase River and local precipitation could be clearly distinguished. In addition, it was possible to identify the occurrence of mountain block recharge using hydrochemistry, specifically the chloride ion. End-member mixing analysis using δ values and Cl⁻concentration revealed spatial variation in the contribution ratios of the river water, mountain block groundwater and local precipitation. Seepage from the Watarase River contributed a significant amount of water to aquifers along its channel. The river-recharged water reached 5 km from the channel in the south (i.e., plain) side and 1.6 km or less in the north (i.e., mountain) side. Remarkable mountain block recharge was observed in the foothills near the axis of the syncline, which has layers of chert and sandstone that likely hinder river channel seepage into the mountain-side aquifers. Major factors controlling the river-water contribution ratio include the distance from the river channel, topography, and hydrogeological settings. The results presented here should facilitate integrated management of groundwater and surface water resources.