Histone lysine demethylase 5 (KDM5) is a ketoglutarate-dependent dioxygenase in histone lysine demethylation, playing a vital role in immunological memory by modulating H3K4me3. Investigating on the role of invertebrate KDM5 in the immune priming, a novel form of immunological memory recently verified in invertebrates, will further our knowledge of epigenetic regulation for innate immune memory. In the present study, a KDM5A was identified in Pacific oyster Crassostrea gigas, and its role in regulating haemocyte proliferation during immune priming was assessed. The deduced amino acid sequence of CgKDM5A harbors a complete JmjC domain featuring a conserved αKG binding site. The mRNA expression of CgKDM5A in the haemolymph was significantly higher than that in the tested tissues of the mature oysters. After Vibrio splendidus stimulation, CgKDM5A transcripts and KDM5 enzymatic activity in haemocytes significantly decreased, accompanying with increased H3K4me3 levels. Moreover, H3K4me3 modifications at the CgBMP7 and CgGATA2/3 promoters were elevated at 7 d after V. splendidus stimulation (p <
0.05), and the haemocyte proliferation index increased significantly at 12 h after the secondary stimulation (p <
0.05). Treatment with KDM5 activator DM-αKG further led to a significant increase in H3K4me3 enrichment levels at the CgBMP7 and CgGATA2/3 promoters at 7 d after the primary stimulation (p <
0.05). Subsequently, the expression of CgBMP7 and CgGATA2/3, as well as the haemocyte proliferation index decreased significantly after the secondary stimulation (p <
0.05). In contrast, CgKDM5A-RNAi oysters exhibited an enriched H3K4me3 at the CgBMP7 and CgGATA2/3 promoters at 7 d after the primary stimulation and an increased haemocyte proliferation index at 12 h after the secondary stimulation (p <
0.05). These findings suggest that CgKDM5A plays a critical role in haemocyte proliferation by affect H3K4me3 enrichment at the CgBMP7 and CgGATA2/3 promoters during immune priming in C. gigas, highlighting the potential of epigenetic regulation in innate immune memory of mollusks.