Conjugated single-layered two-dimensional covalent organic frameworks are flat and extended polymer networks with a unique combination of material properties, giving rise to potential applications in sensing, optoelectronics, and photonics. Despite their great potential, thus far only a few reactions to access such extended conjugated 2D polymers have been reported. Here, the on-surface polymerization of the first vinylene-linked single layered two-dimensional covalent organic framework using reversible Knoevenagel polycondensation under solvothermal conditions is described. Self-assembly of the two monomer building blocks at the solid−liquid interface led to the formation of extended covalent networks at room temperature without the need of additional catalysts or reagents. The described approach grants access to extended conjugated 2D polymers under unprecedentedly mild conditions and paves the way to new hybrid material systems.