Pacific oyster (Crassostrea gigas) is one of the most widely cultivated shellfish species in the world. Because of its economic value and complex life cycle involving drastic changes from a free-swimming larva to a sessile juvenile, C. gigas has been used as a model for developmental, environmental, and aquaculture research. However, due to the lack of genetic tools for functional analysis, gene functions associated with biological or economic traits cannot be easily determined. Here, we reported a successful application of CRISPR/Cas9 technology for knockout of myosin essential light chain gene (CgMELC) in C. gigas. C. gigas embryos injected with sgRNAs/Cas9 contained extensive indel mutations at the target sites. The mutant larvae showed defective musculature and reduced motility. In addition, knockout of CgMELC disrupted the expression and patterning of myosin heavy chain positive myofibers in larvae. Together, these data indicate that CgMELC is involved in larval muscle contraction and myogenesis in oyster larvae.