In view of the local scour problem around pile groups in cohesive sediment beds, the local scour characteristics of a single pile and 5×5 pile groups under various pile spacing conditions were investigated through laboratory flume experiments. In the experiments, 0.038 mm quartz powder was used to simulate silty cohesive sediment. Combined with geotechnical parameter measurements and three-dimensional topographic scanning technology, the variation law of scour depth over time, the integral characteristics of pile group scour, and the influence of pile spacing on scour efficiency were analyzed. Moreover, the differences in scour mechanisms between cohesive and non-cohesive sediments and their physical driving factors were discussed. The results show that the temporal variation of local scour depth in cohesive sediment can be divided into an initial scour stage, a scour development stage, and a scour equilibrium stage. The scour depth behind the pile is generally greater than or equal to that in front of the pile, which is significantly different from the local scour characteristics around piles in non-cohesive sediment. Furthermore, the local scour efficiency of pile groups decreases with the increase in pile spacing; the maximum scour depth reaches nearly six times the pile diameter at three times the pile spacing, which is 2.4 times the local scour depth of a single pile. The local scour depth of the piles in the first three rows is generally smaller than that of the downstream piles, and the scour depth tends to be stable after the third row.