The oceanic uptake of anthropogenic CO₂ has resulted in acidification in surface and subsurface waters throughout the global ocean. We initiated acidification monitoring in the eastern part of the Tsugaru Strait, which connects the Sea of Japan and the North Pacific. Annual mean pH and CaCO₃ saturation state decreased considerably throughout all depths at rates of 0.0030−0.0051 yr⁻¹ and 0.017−0.036 yr⁻¹ during 2012-2019, respectively. These rates of decrease are faster than those caused by increasing atmospheric CO₂, and faster than those observed during previous research in the Sea of Japan and the North Pacific. The accelerated acidification is attributed to enhanced increase in dissolved inorganic carbon caused by elevated mixing of the upper and deeper waters from the Sea of Japan at the western part of the strait by the strengthening of the Tsugaru Warm Current.