Global concerns related to the rise in worldwide energy demand, along with the pledge to minimize greenhouse gas emissions, have encouraged developed nations to opt for clean, efficient, and sustainable nuclear energy. However, the generation of volatile radioactive by-products (such as ¹²⁹I and ¹³¹I) during the operation of a nuclear power plant is considered a serious environmental concern, especially in the case of an accident. As a result, the development of efficient iodine sequestering adsorbents from both aqueous and vapor media is an important contemporary research domain. In this regard, we report herein a guanidinium-based ionic covalent organic network (iCON), denoted as iCON-4, that was prepared using a Schiff-base polycondensation reaction between terephthalaldehyde and triaminoguanidinium chloride. The polymeric iCON-4 is robust and it exhibits good physiochemical stability. The positively charged guanidinium moieties present in iCON-4 facilitate the ion-exchange based adsorption of iodine leading to faster uptake kinetics as well as relatively higher capture capacity. In fact, the iCON-4 polymeric network shows iodine removal greater than 99% from the aqueous medium within two minutes and it shows excellent affinity (distribution coefficient, ∼10⁵ ml g⁻¹) towards iodine in the aqueous medium. Also, it displays fast kinetics during the removal of iodine species from aqueous water samples collected from diverse water bodies such as seas, rivers and lakes. Additionally, iCON-4 registered remarkable selectivity while capturing iodide ions in the presence of other competing anionic species. For practical applications, iCON-4 can be reused up to seven times without any remarkable loss in uptake performance. Thus, iCON-4 has been developed as a strategic material with all desirable attributes required in an adsorbent for practical applications.