Designing reusable high‐performance heterogeneous palladium (Pd) catalysts via convenient, economic synthesis is of great importance to the industrial applications of various carbon‐carbon cross‐coupling reactions. We demonstrate herein a convenient one‐pot self‐encapsulation synthesis of a heterogeneous Pd catalyst [Pd@PDEB, PDEB=poly(1,3‐diethynylbenzene)] directly from commercially available, economic precursors. In the synthesis, the formation of the cross‐linked polymer networks and Pd encapsulation are accomplished simultaneously, turning a homogeneous Pd polymerization catalyst into the heterogeneous cross‐coupling catalyst. As a unique, practical heterogeneous catalyst, Pd@PDEB shows remarkably high activity, high reusability, and high versatility towards at least four types of cross‐coupling reactions (Suzuki–Miyaura, Stille, allylic arylation, and Mizoroki–Heck reactions) with even difficult reactants (aryl chlorides and heteroaryl halides) under aerobic conditions with Pd loadings down to ppm or even ppb levels. Evidences from hot filtration and 3‐phase tests demonstrate the heterogeneous nature of the catalyst with very low Pd leaching and negligible contributions of leached homogeneous Pd species towards the coupling reactions.