Due to the advantages of light-weight, excellent integrated physical/chemical properties and design flexibility, metal/polymer hybrid structures have been increasingly used in automotive, aerospace and biomedical industries, which calls for effective method for joining metal and polymer. In this study, biomedical 3D-printed porous TC4 titanium alloy plate and ultra high molecular weight polyethylene (UHMWPE) plate were successfully joined by friction spot welding (FSpW) technology. The unique welding temperature (Tw) - z-axis force (Fz) evolution at the TC4 plate/UHMWPE plate interface were characterized. During dwell stage, Tw and Fz were identified to be coupled. An arch-shaped embedded zone (EZ) which characterized by the embedment of UHMWPE into porous TC4 formed under non-uniformly distributed Tw at the interface, which provided the most effective bonding mechanism. The porous structure of TC4 plate is critical for the mechanical interfacial bonding formation. As a result, TC4/UHMWPE joints of high lap shear strength can be obtained by forming large EZ. Failure occurred at either TC4 or UHMWPE base plates rather than the TC4 plate/UHMWPE plate interface. Keywords: Friction spot welding, Metal/polymer joining, Welding temperature and force, Macrostructure evolution, Mechanical properties