Ultrasonic melt treatment (UST) was applied to an A390 hypereutectic Al–Si alloy in a temperature range of 750–800 °C and its influence on the solidification structure and the consequent increase in strength was investigated. UST at such a high temperature, which is about 100 °C above the liquidus temperature, had little effect on the grain refinement but enhanced the homogeneity of the microstructure with the uniform distribution of constituent phases (e.g. primary Si, α-Al and intermetallics) significantly refined. With the microstructural homogeneity, quantitative analysis confirmed that UST was found to suppress the formation of Cu-bearing phases, i.e., Q-Al5Cu2Mg8Si6, Al2Cu phases that form in the final stage of solidification while notably increasing the average Cu contents in the matrix from 1.29 to 2.06 wt%. A tensile test exhibits an increase in the yield strength of the as-cast alloy from 185 to 208 MPa, which is mainly associated with the solute increment within the matrix. The important role of UST in the microstructure evolution during solidification is discussed and the mechanism covering the microstructure-strengthening interrelationship of the ultrasonically treated A390 alloy is proposed.