A series of colloidal M x Fe3–x O4 (M = Mn, Co, Ni; x = 0–1) nanoparticles with diameters ranging from 6.8 to 11.6 nm was synthesized by hydrothermal reaction in aqueous medium at low temperature (200 °C). Energy-dispersive X-ray microanalysis and inductively coupled plasma spectrometry confirm that the actual elemental compositions agree well with the nominal ones. The structural properties of the obtained nanoparticles were investigated by powder X-ray diffraction, Raman spectroscopy, Mössbauer spectroscopy, X-ray and neutron pair distribution function analysis, and electron microscopy. The results demonstrate that our synthesis technique leads to the formation of chemically uniform single-phase solid solution nanoparticles with cubic spinel structure, confirming intrinsic doping. The local structure of the Fe3O4 NPs is distorted with respect to the cubic inverse-spinel structure, while chemical substitution of Fe by Mn or Ni partially eliminates the local distortions. Magnetic studies showed that, in comparison to nondoped Fe3O4, the saturation magnetization (M s) of M x Fe3–x O4 (M = Mn, Ni) decreases with increasing dopant concentration, while Co-doped samples showed similar M s. On the other hand, whereas Mn- and Ni-doped nanoparticles exhibit superparamagnetic behavior at room temperature, ferrimagnetism emerges for Co x Fe3–x O4 nanoparticles, which can be tuned by the level of Co doping.