The Underwater Sonar Moving Target (USMT) dataset is a forward-looking sonar (FLS) image dataset for low-pixel underwater moving-target perception and segmentation. It contains 21 temporally ordered sequences and 7,136 annotated rectangular B-scan FLS images collected during AUV experiments in a controlled lake environment. Each sonar image is accompanied by a pixel-level annotation in LabelMe JSON format and a TXT file containing the corresponding sonar acquisition metadata. The annotations were generated using X-AnyLabeling with the SAM 2 video model and subsequently reviewed and refined manually. The dataset can support research on underwater intelligent perception, sonar image segmentation, and temporal modeling of moving targets.