Cyclic GMP-AMP synthase (cGAS) is a critical intracellular sensor that can recognize abnormally located DNA in the cytoplasm and trigger immune responses. To elucidate the critical role of cGAS in the regulation of innate immunity in mollusks, we successfully cloned and analyzed Crassostrea gigas cGAS (CgcGAS). The open reading frame (ORF) of CgcGAS was 1623bp and encoded 540 amino acids with a theoretical molecular weight of 62.3 kDa and a conserved Mab21 domain. Phylogenetic analysis confirmed that CgcGAS was a member of the molluscan cGAS family. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) results revealed widespread expression of CgcGAS in various tissues, with the highest relative expression in the digestive glands. Subsequently, subcellular localization experiments showed that CgcGAS was observed in both the nucleus and cytoplasm, with a predominant nuclear localization, suggesting that CgcGAS may have played a role in DNA sensing in the nucleus and DNA binding and signaling in the cytoplasm. Furthermore, dual-luciferase reporter gene assays and RNA interference experiments revealed that CgcGAS could activate the NF-κB and ISRE signaling pathways, as well as the expression of downstream inflammation-related factors, such as virus inhibitory protein endoplasmic reticulum-associated interferon-inducible (viperin), tumor necrosis factor (TNF), interleukin-17 (IL-17), and the transcription factor interferon regulatory factor 2/8 (IRF2/8). In conclusion, CgcGAS played a critical role in the signal transduction process of innate immune responses in Crassostrea gigas.