The present work describes the preparation of a new covalent organic polymer (COP) and its application as a hetero support for diaryl selenides synthesis. A nitrogen rich COP (CGP) has been synthesized via SNAr reaction of cyanuric chloride with guanidinium hydrochloride. The successful confinement of COP with Ni nanoparticles through post-synthetic transformations (Ni@CGP) provides excellent catalytic activity for the transformation of aryl halides into diaryl selenides using elemental selenium powder. The synthetic transformations are well confirmed using various modern analytical and spectroscopic techniques which reveal high chemical and thermal durability. The N-rich framework of CGP fortifies the confinement of Ni NPs. Ni@CGP provides an efficient approach for diaryl selenides synthesis using a very cheap selenating reagent under water benign solvent conditions (DMSO : H2O) at room temperature with high reusability. Significantly, our work not only contributes the opportunity for developing economical and effective non-noble metal decorated COPs as heterogeneous catalysts, but also delivers an efficient approach to produce industrially important C-Se coupling products.