Precise fabrication of porous ternary Pt-based nanodendrites is very important for electrochemical energy conversion owing to high surface area and great molecular accessibility of these nanodendrites. Herein, PtPdRu porous nanodendrites (PNDs) were prepared via a facile one-step ultrasonic irradiation approach at room temperature. Intriguingly, the ultrasonic irradiation drove the formation of PtPdRu PNDs with spatially interconnected porous structures, whereas magnetic stirring produced PtPdRu nanoflowers (NFs) with less porosity. The formation mechanism was ascribed to the acoustic cavitation effect and fast-reduction kinetics under sonication. The as-made PtPdRu PNDs displayed a superior catalytic performance towards ethanol oxidation reaction with a high tolerance for CO-poisoning as compared to PtPdRu NFs, PtPd NDs, and commercial Pt/C catalyst.