Remedial grouting under existing dams founded on rock is often conducted under unfavorable hydraulic conditions with high gradients and fast-flowing water in the rock fractures. The fast-flowing water may push the grout downstream, change the shape of the grout spread, and erode the fresh grout. All these issues can jeopardize the outcome of the grouting work. To deal with the fast-flowing water, drilling pressure relief holes upstream of the grout holes can be one of the alternatives. These pressure relief holes function as additional drainage. They can reduce the velocity of the water in the fractures near the grout holes, thus reducing the risk of erosion of the grout. However, the usage of pressure relief holes in remedial grouting practices has not been widely documented, and different opinions still exist on their effectiveness. Meanwhile, few studies have analyzed the effectiveness of the relief holes. In this paper, the usage of pressure relief holes in remedial grouting of rock foundations under existing dams is analyzed. The effectiveness of the relief holes is evaluated using both analytical calculations and numerical simulations. The analysis results show that the pressure relief holes can reduce the downstream hydraulic gradient and facilitate the remedial grouting. Based on these results, discussions are made on their effectiveness together with suggestions on how to deal with excessive groundwater flow in remedial grouting.