Magnetic iron-oxide nanoparticles exhibit high efficiency in wastewater treatment for many important reasons, including that they can remove contaminants from wastewater rapidly owing to their high external surface area/unit mass and interstice reactivity. Additionally, this type of iron oxide can easily be separated using a magnet after finishing the treatment process, can be used as a catalyst for the decomposition of adsorbed contaminants and thus reduce sludge formation, and can cost-effectively meet the environmental regulations for wastewater treatment since it can be prepared in situ where treatment is needed via various techniques. In this study, we use magnetic iron oxide nanoparticles for dye removal from synthetic and real textile wastewater for the first time. The effects of different experimental parameters on dye removal, such as contact time, initial concentration, solution pH, and coexisting ions, were investigated. Computational modelling of the interaction of different dye molecules with different surfaces of -Fe2O3 nanoparticles is performed to obtain more mechanistic insights on the adsorption behaviour. The results showed that dye adsorption was fast, as external adsorption was dominated. The adsorption equilibrium data fit very closely to the Langmuir adsorption isotherm model, confirming monolayer adsorption, which is supported by the adsorption computational calculations. The adsorption was spontaneous, endothermic, and physical in nature.