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An effective external prestressing technique is presented for the seismic retrofit of reinforced concrete beam–column joints with prestressed high-strength steel wires. This method is to improve the seismic performance and enhance the shear strength of beam–column joints with non-seismic design as well as to overcome the shortcomings of traditional seismic retrofitting techniques for the joints. Six large-scale interior beam–column joints were tested under reversed cyclic loading, where five were strengthened by the proposed technique and one was un-retrofitted as a control specimen. Particular emphasis was placed on the effects of the ratio of prestressed high-strength steel wires, prestressing level of wires and polymer grout layer. Test results showed that the joint shear capacity was increased significantly and cracks in beam–column joints were controlled effectively after retrofitting. All retrofitted joint specimens failed in a ductile mode of beam flexural failure, whereas the control specimen failed in a brittle mode of joint shear failure. In addition, the energy dissipation capacity as well as ductility of the retrofitted joint specimens was enhanced significantly compared with the un-retrofitted specimen.