Condition-specific competition, a phenomenon in which competition between species is mediated by physical environmental factors, considered to be an important element in explaining species coexistence. In Hokkaido, Japan, Dolly Varden and white-spotted charr are distributed in the same river with a narrow sympatric zone and is believed to be the result of temperature‐mediated competition. Previous experimental studies suggest that white-spotted charr have an advantage at higher water temperatures; however, the dominance of Dolly Varden in lower water temperatures has not been proven. To address this issue, we conducted a field survey in 30 tributaries of the Sorachi River including some tributaries as controls to separate the water temperature effects and species interactions. We found that the occurrence of white-spotted charr increased with increasing water temperature. In the control tributaries with few Dolly Varden, white-spotted charr dominated with high densities despite lower water temperatures, while Dolly Varden dominated in the naturally isolated tributaries (i.e., less influence of white-spotted charr) despite higher water temperature. This not only supports the results of former studies but also suggests the dominance of Dolly Varden in lower water temperature. We therefore revealed a reversal of interspecific dominance by temperature-dependent competition in the field.