<p>Ultrasonic welding is commonly used for joining thermoplastic composites, but ensuring the strength and stability of the welding interface presents a significant challenge in practical applications. This study employed ultrasonic welding to develop robust ASA and ASA/PMMA joints and evaluated a welding approach for thermoplastic composites that could operate without traditional energy directors (ED). This strategy utilized polyethersulfone (PES) as an interlayer at the welding interface, coupled with plasma-treated substrates, to achieve significantly enhanced bonding strength for high-performance ultrasonic welding. The ultrasonic welded joints were analyzed using Fourier Transform Infrared (FTIR) spectroscopy, single-lap shear test, interface analysis, and wettability test. The results demonstrated that the PES interlayer significantly reduced the standard deviation of the single-lap shear strength (SLSS), altered the bonding mechanism at the welding interface, and avoided the poor direct weld compatibility issues between dissimilar materials. Furthermore, the plasma treatment promoted the adhesion of molten PES to the welding interface, while the ultrasonic vibration facilitated its uniform dispersion across the interface, thereby enhancing the interfacial bonding strength. Consequently, the plasma-assisted PES interlayer provides a reliable joining technology for ultrasonic welding applications.</p>