Metal–plastic composite tubular structures that combine the high strength and stiffness of metallic tubes with the lightweight and flexible properties of plastic tubes, exhibit considerable potential to provide lightweight structures with improved performance compared to conventional monolithic tubular parts. However, the lack of well-confirmed technologies for connecting metal–plastic composite tubes that can ensure good structural stability and strength has proven problematic in actual applications. In this study, an innovative mechanical joining process via a hydroformed threaded coupling is proposed to achieve the successful joining of a metal–plastic composite layered tube (CLT). For the threaded connection, the CLT with an internal thread to act as a sleeve and the coupling tube with an external thread to act as a screw were joined together. A three-layer CLT composed of AISI 304 stainless steel/polyvinyl chloride (PVC)/AISI 304 stainless steel was investigated by free bulging and thread hydroforming. For successful forming into a threaded CLT, the optimal loading path was designed using analytical forming pressure and numerical contracting stroke. The mechanical tests based on compression and lateral three-point bending tests provided several key indicators for the quantitative performance of the thread-coupled parts. The reliability and applicability of the threaded coupling confirmed that the proposed joining process has strong potential for successful utilization in connecting metal–plastic composite tubes for structural applications.