The structural and magnetic properties of Nd0.1Sr0.9FeO3-δ nanoparticles (∼60–70 ​nm) formed by mechanosynthesis at 800 ​°C (12 ​h), which is ∼ 200–600 ​°C lower than the reported formation temperatures of rare earth cation-doped SrFeO3-δ phases, are reported. Both XRD and 57Fe Mӧssbauer spectroscopy show the nanoparticles to be a mixture of the cubic Nd0.1Sr0.9FeO3 and the Fe4+/Fe3+ mixed-valence tetragonal Nd0.1Sr0.9FeO2.875 phases in the ratio of ∼ 45 : 55. XPS data reveal a complex surface composition for the nanoparticles where traces of the initial reactants as well as other SrFeO3-δ phases are detected. The Nd0.1Sr0.9FeO3-δ nanoparticles are found to display superparamagnetism with blocking temperatures of ≥78 ​K for the Nd0.1Sr0.9FeO3 phase and ∼ 61 ​K for the Nd0.1Sr0.9FeO2.875 one. The nanoparticles exhibit an antiferromagnetic-to-weak ferromagnetic transition at ∼23 ​K which is attributed to the combined effects of the helical magnetic structure of Nd0.1Sr0.9FeO3, spin canting in Nd0.1Sr0.9FeO2.875 and doping with the Nd3+cation. Display omitted •Nd0.1Sr0.9FeO3-δ nanoparticles (∼60–70 ​nm) were formed by mechanosynthesis at 800 ​°C.•The nanoparticles were found to consist of cubic Nd0.1Sr0.9FeO3 and tetragonal Nd0.1Sr0.9FeO2.875 in the ratio of ∼ 45 : 55.•XPS reveals a complex surface composition where traces of the initial reactants are detected.•The nanoparticles are superparamagnetic with TB ≥78 ​K and TB∼ 61 ​K for the cubic phase and tetragonal one, respectively.•An antiferromagnetic-to-weak ferromagnetic transition is observed at ∼23 ​K.