Optical ultrasonic probes, exemplified by Fabry–Perot cavities on optical fibers, have small sizes, high sensitivity, and pure optical characteristics, making them highly attractive in high-resolution ultrasonic/photoacoustic imaging, especially in near-field or endoscopic scenarios. Taking a different approach, we demonstrate an ultrasensitive and broadband ultrasound microprobe formed by an optical whispering-gallery-mode polymer microcavity coupled to a U-shaped microfiber. With the high-quality (Q) factors (>106), the noise equivalent pressure of the ultrasound microprobe reaches 1.07mPa/√Hz with a record broadband response of 150 MHz and a large detection angle of 180°. Our results show that this optical microprobe can overcome the strong decay resulting from ultrasound diverging and medium absorption through short working distances. We further demonstrate high-quality in vivo whole-body photoacoustic imaging of a zebrafish larva. Our implementation provides a new strategy for developing miniature ultrasound detectors and holds great potential for broad applications.