Mechanical properties and corrosion resistance of La₂O₃/A356 composites with different contents of La₂O₃ were investigated by optical microscopy, X-ray diffractometry, scanning electron microscopy, electrochemical tests, and immersion corrosion tests. The results show that the addition of La₂O₃ refined the α-Al phase of the A356 matrix, and the long stripe-like Si phase and β-Al₅FeSi phase were transformed into short rod-like forms. The La₂O₃/A356 composites with 1.0 wt.% La₂O₃ exhibited the most optimal mechanical properties and corrosion resistance. The yield strength, ultimate tensile strength, and elongation of La₂O₃/A356 composites with 1.0 wt.% La₂O₃ were higher than those of the matrix. The results of electrochemical experiments and the immersion corrosion test show that the corrosion potential of La₂O₃/A356 composites with 1.0 wt.% La₂O₃ was 72 mV higher than that of the matrix, the corrosion current density was 84.8% lower than that of the matrix, and the impedance Z was improved by 59.1% compared to the matrix. The addition of La₂O₃ improved the mechanical properties of the A356 matrix by refining the grains, inhibiting the nucleation of eutectic Si, and promoting the twinning growth mechanism. Moreover, the effect of La₂O₃ on the micro-galvanic corrosion behavior of A356 was discussed.