The development of novel methods for industrial production of metal-matrix composites with improved properties is extremely important. An aluminum matrix reinforced by “in situ” α-Al₂O₃ nanoparticles was fabricated via direct chemical reaction between molten aluminum and rutile TiO₂ nanopowder under the layer of molten salts at 700–800 °C in air atmosphere. Morphology, size, and distribution of the in situ particles, as well as the microstructure and mechanical properties of the composites were investigated by XRD, SEM, Raman spectra, and hardness and tensile tests. Synthesized aluminum–alumina composites with Al₂O₃ concentration up to 19 wt.% had a characteristic metallic luster, their surfaces were smooth without any cracks and porosity. The obtained results indicate that the “in situ” particles were mainly cube-shaped on the nanometer scale and uniform matrix distribution. The concentration of Al₂O₃ nanoparticles depended on the exposure time and initial precursor concentration, rather than on the synthesis temperature. The influence of the structure of the studied materials on their ultimate strength, yield strength, and plasticity under static loads was established. It is shown that under static uniaxial tension, the cast aluminum composites containing aluminum oxide nanoparticles demonstrated significantly increased tensile strength, yield strength, and ductility. The microhardness and tensile strength of the composite material were by 20–30% higher than those of the metallic aluminum. The related elongation increased three times after the addition of nano-α Al₂O₃ into the aluminum matrix. Composite materials of the Al-Al₂O₃ system could be easily rolled into thin and ductile foils and wires. They could be re-melted for the repeated application.