In this paper, temperature-dependent magnetic properties of magnetite nanoparticles (MNPs) synthesized at room temperature via chemical coprecipitation were investigated. Monodispersed MNPs with an average size of 11.22 nm were synthesized at a stirring rate of 200 rpm and a pH value of 10. The phase composition, particle size/morphology, and magnetic properties of the synthesized nanoparticles were characterized by X-ray diffractometer (XRD), scanning transmission electron microscope (STEM) and superconducting quantum interference device (SQUID), respectively. The results show that the synthesized nanoparticles are magnetite with a spinel structure without any impurities. The synthesized MNPs exhibit a nearly spherical morphology with a uniform particle size distribution and an average particle size of 11 nm. Based on the analysis of the temperature-dependent magnetic properties at 5–300 K, the coercivity and remanence in the hysteresis loop for the MNPs decrease with the increase in temperature and disappear at room temperature (i.e., 300 K), thus showing a superparamagnetic behaviour. The superparamagnetic behaviour of the synthesized MNPs was analysed using a developed model based on the core-shell theory. The experimental results of magnetization are in a reasonable agreement with the calculated data by the developed model. The magnetic susceptibility of the synthesized MNPs increases and the coercivity, exchange bias, remanent and saturation magnetizations decrease with the increase of the temperature. It is indicated that the synthesized MNPs could be used as a promising candidate in various applications such as heat transfer, magnetic hyperthermia and magnetic separation.