AbstractLithium‐sulphur (Li−S) batteries are a promising alternative power source, as they can provide a higher energy density than current lithium‐ion batteries. Porous materials are often used as cathode materials as they can act as a host for sulphur in such batteries. Recently, covalent organic frameworks (COFs) have also been used, however they typically suffer from stability issues, resulting in limited and thus insufficient durability under practical conditions and applications. Herein, we report the synthesis of a crystalline and porous imine‐linked triazine‐based dimethoxybenzo‐dithiophene functionalized COF (TTT‐DMTD) incorporating high‐density redox sites. The imine linkages were further post‐synthetically transformed to yield a robust thiazole‐linked COF (THZ‐DMTD) by utilizing a sulphur‐assisted chemical conversion method, while maintaining the crystallinity. As a synergistic effect of its high crystallinity, porosity and the presence of redox‐active moieties, the thiazole‐linked THZ‐DMTD exhibited a high capacity and long‐term stability (642 mAh g⁻¹ at 1.0 C; 78.9 % capacity retention after 200 cycles) when applied as a cathode material in a Li−S battery.