Most of the existing photoacoustic imaging systems detect ultrasound signals using piezoelectric transducers. As the performance of these transducers largely degrades with decreasing size, optical ultrasound sensors have recently attracted growing interest. Unlike piezoelectric transducers, miniaturized optical ultrasound sensors typically provide a wide bandwidth and high sensitivity almost independent of size. Therefore, they can potentially realize deeper and higher-resolution photoacoustic imaging than the existing systems. This article reviews recent developments in miniaturized optical ultrasound sensors and briefly compares three types of miniaturized optical ultrasound sensors and methods of parallel interrogation. We also introduce some recent biomedical demonstrations of photoacoustic imaging using optical ultrasound sensors. Finally, the article discusses future research and development trends in optical ultrasound sensors.