Superparamagnetic nanoparticles of zinc ferrite ZnFe₂O₄ were obtained by air heat treatment of iron and zinc sulfated layered double hydroxide (SLDH) Zn₂Fe₄(OH)₁₂SO₄·8H₂O at 600°С for 1 h. The initial sulfated hydroxide was obtained on a silicon substrate as a result of 30 cycles of successive ionic layer deposition from 0.01 M solutions of the Mohr salt (NH₄)₂Fe(SO₄)₂ and zinc ammonia complex [Zn(NH₃)₄](NO₃)₂. Iron and zinc SLDH nanocrystals have the nanosheets morphology with an average thickness of 3–7 nm and a diameter of about 17 nm which are oriented perpendicular to the silicon substrate plane. Physico-chemical processes which occur during thermal treatment (25–600°С) of iron and zinc sulfated layered double hydroxide and lead to the formation of zinc ferrite at 550°С are investigated by simultaneous thermal analysis. It was found that ZnFe₂O₄ nanocrystals with an average size of about 30 nm and isometric morphology, which partially preserve the morphological motifs of the original SLDH nanosheets, were formed as a result of the thermal treatment of sulfated hydroxide. The obtained ZnFe₂O₄ nanocrystals are chemically and phase-pure, have lattice parameters a = b = c = 8.4394 Å and behave like superparamagnetic nanoparticles: they are characterized by the magnetization of 1.85 (298 K) and 12.79 (77 K) emu/g with 18 kOe applied field, and the effective magnetic moment is μ_eff = 24.3 μB per formula unit. This phase-pure superparamagnetic ZnFe₂O₄ nanoparticles can be potentially used as the basis for magnetic fluids, gas sensors, drugs for hyperthermia and magnetic resonance imaging diagnostics.