Ferrous - ferric ions and surface functional groups on iron oxide nanoparticles (NPs) can be tailored to attain requisite magneto-thermal properties propitious for hyperthermia mediated tumor ablation. We report the synthesis of polyvinylpyrrolidone (PVP) grafted octahedron shaped iron oxide NPs via a facile hydrothermal route for magnetic fluid hyperthermia. PVP grafted nano-octahedrons are being first time used for the morphology dependent hyperthermia study. Physico-chemical, magnetic and thermal properties have been systematically characterised in the present study. Aqueous suspension of nano-octahedrons has displayed excellent colloidal stability with a hydrophilic nature. Magnetic nano-octahedrons are found to possess nearly superparamagnetic behaviour with a saturation magnetisation value of ∼ 82 emu/g and very low coercivity of ∼ 138 Oe pertinent for hyperthermia applications. A high specific absorption rate (SAR) value of ∼ 963 W/g under an AC magnetic field of 600 Oe and 316 kHz has been obtained with concentration of 5 mg/mL which is predominantly attributed to hysteresis and effective relaxation losses. In order to elucidate the role of viscosity on Brownian relaxation, a detailed analysis of the amount of heat dissipation from nano-octahedrons in different aqueous media such as water, ethylene glycol (EG) and agar solution have also been investigated. Viscosity of these ferrofluids has been observed to be strongly dependent on the Brownian relaxation consequently changing the SAR values. Moreover, nano-octahedrons are biocompatible and exhibit non-hemolytic activity. © 2020 Elsevier B.V.