In the present work, the crystallization of anatase TiO₂ nanoparticles (NPs), using different ferrite nanoparticles with different chemical composition, dimensions and shape as nucleation seeds, was investigated. In particular, CoFe₂O₄, NiFe₂O₄ and Fe₃O₄ NPs with a volume ratio equal to 1:1000 with respect of TiO₂ amount, were used in order to investigate the synthesis of nanocrystalline tetragonal anatase TiO₂ by a hydrothermal synthesis. In addition, Lu₂O₃ nanoparticles were also used to detect the effect of a non-magnetic nanoparticle on the synthesis and nanocrystallization of titania. For each sample, a deep physical characterization was performed by XRD (with a Rietveld refinement of the structural data), FE-SEM, STEM, HRTEM, DSC analysis and BET surface area measurement. Furthermore, for some samples, the photocatalytic activity was investigated by degradation of methylene blue in aqueous medium, in the framework of a standard ISO 10678:2010 protocol. The hydrothermal synthesis was performed with a 3 hours' thermal treatment, at a pressure of approximatively 9 bar and a temperature significantly lower (Tmax═150 °C) than the usual temperature necessary to obtain crystalline anatase TiO₂ (Tcryst═350 °C). The results give evidence that the mere presence of a nucleation seeds in the hydrothermal reactor, without any particular need for the composition and morphology, leads to crystalline anatase TiO₂ nanoparticles with high photocatalytic performances.