Interaction of acoustic waves and microbubbles occurs in numerous biomedical applications including ultrasound imaging, drug delivery, lithotripsy treatment, and cell manipulation, wherein the acoustically driven microbubbles routinely act as active microscale oscillators or actuators. In contrast, microbubbles were utilized here as passive receivers to detect broadband ultrasound waves in aqueous environments. The microbubble was photothermally generated on a microstructured optical fiber (MOF) tip, forming a flexible Fabry–Pérot cavity whose gas–water interface was sensitive to ultrasound waves. The MOF severed as both a low-loss waveguide and a compact light condenser, allowing high-efficiency generation and stabilization of ultrasmall microbubbles. Integrated with all-fiber interferometry, a 10 μm diameter microbubble exhibited a low noise-equivalent pressure level of $～3.4\mathrm{mPa}/{\mathrm{Hz}}^{1/2}$ and a broad bandwidth of $～0.8\mathrm{MHz}$, capable of detecting weak ultrasounds emitted from red blood cells irradiated by pulsed laser light. With advantages of high sensitivity, compact size, and low cost, the microbubble-based ultrasound sensor has great potential in biomedical imaging and sensing applications.