AbstractThe synthesis of porphyrinoid‐based low‐dimensional polymers has recently attracted considerable interest in view of their intriguing electronic, optical, and catalytic properties. Here, this is introduced by the surface‐assisted synthesis of two carbaporphyrinoid‐based polymers of increasing dimensionality under ultrahigh‐vacuum conditions. The structural and electronic characterization of the resulting polymers has been performed by scanning tunneling and non‐contact atomic force microscopies, complemented by theoretical modeling. First, a carbon‐carbon coupling between dicarbahemiporphyrazine precursors is achieved by thermal activation of their isopropyl substituents via a [3+3] cycloaromatization, giving rise to one‐dimensional (1D) polymers. Second, the same precursor is functionalized with chlorine atoms to complement the [3+3] cycloaromatization with orthogonal dehalogenation and homocoupling, affording two‐dimensional (2D) molecular nanostructures. In addition, both low‐dimensional free‐base porphyrinoid‐based polymers are exposed to an atomic flux of cobalt atoms, giving rise to cobalt‐metalated macrocycles, with the metal atoms coordinated only to the two pyrrolic nitrogens, in contrast to the typical four‐fold coordination that occurs inside tetrapyrroles. This on‐surface protocol renders atomically precise covalently‐linked porphyrinoid polymers and provides promising model systems toward the exploration of low‐coordinated metals with utility in diverse technological areas.