Abstract: Chitinase, as a crucial enzyme for the degradation of chitin, is involved in the construction of the chitin framework during the process of shell formation. In order to identify the members of the chitinase gene family in Crassostrea gigas and investigate their response to acidification, bioinformatic methods were employed to identify the chitinase family members and analyze their expression patterns. Eleven members of the chitinase family were identified from the C. gigas genome. All gene members contained the Glyco-18 domain, and some genes also contained the chitin-binding domain ChtBD2. These genes were predominantly located on chromosome 2, 5, 6, and 7. In the C. gigas, the chitinase family genes were clearly divided into two branches which were endochitinases and exochitinases. The chitinase family expressed across all developmental stages of the C. gigas larvae. With the development of larva, the expression level of five genes increased gradually. The expression levels of most chitinase family genes were higher in the mantle compared to other tissues. The acidic mammalian chitinase (Cgamcase-1) exhibited high expression level in the mantle, with the highest expression level in the outer fold (OF). The expression patterns of Cgamcase-1 in response to acidification were analyzed. After 3, 7, and 14 days of acidification stress, the mRNA expression of Cgamcase-1 in the mantle was 3.010-fold (P < 0.05), 4.557-fold (P < 0.001) and 4.129-fold (P < 0.001) of that in the control group, respectively. After 7 days of acidification stress, the mRNA expression of Cgamcase-1 in OF was 3.598-fold of that in the control group (P < 0.05). In situ hybridization results revealed that the positive signals for the Cgamcase-1 probe were primarily concentrated in the epithelial cell region of the outer fold, and the intensity of the positive signals significantly increased after 7 days of acidification stress, while it significantly decreased after 14 and 28 days. The study suggested that chitinase family genes might be involved in the process of larval development and adult shell formation. Cgamcase-1 participated in chitin degradation and responding to ocean acidification. This research provided important theoretical evidence and reference for understanding the role of chitinase in the shell formation process of the C. gigas and their response mechanisms under ocean acidification.