Major element, trace element, and Sr–Nd–Pb isotopic compositions of high‐potassium calc‐alkaline rhyolites (the T6 rhyolites) collected from the western slope of the middle Okinawa Trough (MOT) are reported in this article. These high‐potassium rhyolites have similar major and trace elemental compositions as Type 2 rhyolites from the MOT, except that the former contain higher concentrations of K2O, Th, U, and Rb and lower concentrations of Sr and Eu. Experimental data indicate that the T6 rhyolites cannot be sourced from low‐ to mid‐potassium MOT rocks by either fractional crystallization or partial melting processes but are instead derived from a cryptic potassium‐rich source beneath the Okinawa Trough (OT). Considering that high‐potassium volcanic rocks are distributed throughout the East China Sea Shelf, the Ryukyu Arc, and Kyushu Island, the high‐potassium OT rocks could represent an important connection between the OT and its adjacent regions. Compared to volcanic rocks from the MOT axial zone, the T6 rhyolites have more depleted Sr–Nd isotopic compositions (87Sr/86Sr = 0.7035–0.7036, 143Nd/144Nd = 0.51291–0.51297) but much more enriched compositions of 207Pb (207Pb/204Pb = 15.60–15.63) and 208Pb (208Pb/204Pb = 38.45–38.57), thus recording a significant DUPAL‐like signature. These isotopic anomalies cannot be explained by the incorporation of subducting sediments or crustal contamination but instead imply the presence of a DUPAL‐like mantle source beneath the OT. The decrease of the DUPAL‐like anomaly from the T6 rhyolites to the MOT axial zone volcanic rocks is consistent with the injection of an normal mid‐ocean ridge basalt (N‐MORB) mantle source, such as asthenospheric material, into pre‐existing DUPAL‐like mantle during back‐arc extension.