Based on the HMPA ligand, a new air- and moisture-stable pentagonal-bipyramidal DyIII single-ion magnet [Dy(HMPA)2(H2O)5]2·Br6·2HMPA·2H2O (1) was prepared and characterized. Single-crystal X-ray tests showed the two crystallographically independent DyIII ions located in a pentagonal-bipyramidal coordination sphere with strong axial HMPA ligands and weak equatorial water molecules relating to strong unaxial anisotropy. Direct-current and alternating-current magnetic susceptibilities were measured and showed that 1 exhibited slow relaxation of magnetization up to 36 K (1000 Hz) with an energy barrier of 556 K and blocking temperature of 7 K (defined by the peak temperature of zero-field cooling data). The application of a dc field and magnetic dilution were also carried out to explore the existing quantum tunelling of the magnetization process.