Achieving rapid and adaptive locomotion in soft robots is essential for navigating complex environments and enabling diverse real-world functions. Here, we present a jellyfish-inspired magnetic soft robot (J-MSR) capable of ultrafast swimming and seamless multimodal motion transitions in liquid environments. By employing an asymmetric trapezoidal magnetic field waveform for actuation, the J-MSR capitalizes on spatial and temporal asymmetries during its swimming cycle, mimicking the natural propulsion mechanism of jellyfish. Through magnetic-fluid-solid multiphysical field coupling analysis and magnetic field waveform optimization, the J-MSR achieves a remarkable swimming speed of 14.85 body lengths per second, demonstrating notably enhanced propulsion performance compared with previously reported jellyfish-inspired robots. Unlike traditional designs relying on auxiliary buoyancy structures, the J-MSR demonstrates versatile multimodal motions under natural negative buoyancy conditions, including large-angle multidirectional swimming (0° to 122°), slit traversal, and rolling. Meanwhile, its exceptional locomotion capabilities facilitate the integration of functional devices, enabling it to perform diverse tasks such as emitting light to mimic fluorescent jellyfish, capturing objects, injecting microneedles, and conducting gastroscopy. These capabilities highlight the J-MSR’s substantial potential as a versatile platform for biomedical applications in confined and unstructured environments.