AbstractSpontaneous electroless deposition of ultrafine metal nanoparticles (NPs) supported on covalent organic frameworks (COFs) for catalytic organic transformation is rarely reported. Herein, poly(phenylene butadiynylene)s (PPBs), a easily prepared two‐dimensional (2D) covalent organic frameworks (COFs) consisting of sp‐ and sp2‐hybridized carbon atoms, can be applied as the reducing agent and substrate for spontaneous electroless deposition of sub‐3 nm ultrafine palladium nanoparticles owing to its low reduction potential and highly conjugated electronic structure. The as‐formed Pd/PPBs heterogeneous catalyst exhibits excellent recyclability and superior catalytic activity towards hydroxycarbonylation of aryl iodides in the presence of carbon monoxide and water.