A novel one-dimensional CoII coordination polymer (OPD)2CoIISO4 was formed from alternating tetrahedral sulphate anions, Co centers and molecules of 1,2-phenylenediamine (OPD). The thermal stability of the structure was confirmed up to ∼230 °C using thermogravimetry and non-ambient powder X-ray diffraction in air. The distorted pseudo-octahedral coordination sphere of CoII ions promotes strong magnetic anisotropy. Therefore (OPD)2CoIISO4 was subjected to thorough characterization with ab initio and DFT calculations along with dc magnetic measurements both of which point to strong easy-axis anisotropy (D/kB≈-87 K, E/kB≈ 23 K). The analysis of ac susceptibility revealed field assisted magnetic relaxation in two field regimes: a low field one with one relaxation time and a high field one where three relaxation times were distinguished. The main role in relaxations of the fastest process (τ1) was attributed to the direct and Raman processes.