We employ an on-surface assembly protocol to synthesize a single layer of a two-dimensional conjugated network (Ni3(HITP)2) on a Au(111) surface. The electronic coupling between the π orbital of the diimine ligand and the d orbital of the metal ion renders efficient π-conjugation. Density-functional theory calculations provide evidence of a non-trivial topological gap in the surface-adsorbed single layer. This work demonstrates that single-layer 2D metal-organic frameworks adsorbed on surfaces are a new class of 2D materials that host quantum phases.