Nanosecond UV laser-induced surface damage of potassium dihydrogen phosphate samples was investigated and discussed by means of defect characterization, in situ damage test, as well as pump–probe shadowgraph imaging. Two distinctive types of surface damage induced by different defects have been demonstrated. Surface damage occurring at relative lower fluence (typically below 5  J  /  cm² in our experiment) is highly correlated with fluorescent surface defects, which are considered as fractural structures introduced by surface cutting. The other type of surface damage that always occurs at higher fluence (above 8  J  /  cm²) is confirmed to originate from the bulk damage precursor located near the crystal surface.