<p>Linkages in covalent organic frameworks (COFs) are the strong covalent bonds that bind molecular building units into porous frameworks. As the stability and properties of the COF structure are often dominated by its linkage, development of suitable linkage chemistry is important. Among the diversity of COF linkages, carbon–carbon bonded linkages are especially interesting for their ultrahigh stabilities. However, due to the highly irreversible nature of carbon–carbon bonds, crystallizing the corresponding COFs remains a major challenge to further expand the scope of this chemistry. Here we develop a postsynthetic linkage conversion strategy for realizing coumarin linkages wherein fused aromatic six-membered rings are constituted by carbon–carbon and carbon–oxygen bonds. With this strategy, five coumarin-linked COFs were synthesized with consecutive Knoevenagel condensation and Pinner reactions. The structures of these COFs were confirmed by powder X-ray diffraction, Fourier-transform infrared spectroscopy and ¹³C solid-state nuclear magnetic resonance spectroscopy. These COFs showed good porosity (Brunauer–Emmett–Teller surface area up to 981 m² g⁻¹), outstanding chemical stability in strong acids and bases and high thermal stability up to around 600 °C. The visible-light-induced green luminescence of one of the COFs was also characterized by photoluminescence spectroscopy.</p><p></p>