Analysis of the hydrographic data taken in the northern North Pacific has revealed a water‐mass front defined as the abrupt change of mixing rates between the two typical water masses. Typical water masses are situated above the main thermocline in the subtropical and subpolar gyres. This front, termed Water‐Mass Front in the present study, can be regarded as the boundary between the subtropical gyre and the subpolar gyre. The index for detecting the frontal position from the temperature field is given as the 5°C isotherm at the 300‐m level. Comparisons with data from previous studies are also made, and this front appears to correspond to the Subarctic Front in almost every in