Abstract: To investigate whether protein kinase D (PKD) plays a regulatory role in inflammatory responses and endoplasmic reticulum (ER) stress in large yellow croaker (Larimichthys crocea), this study established both in vivo and in vitro inflammation models based on preliminary experiments and relevant literature. This was achieved by incubating large yellow croaker head kidney macrophages with 50 ng/μL lipopolysaccharide (LPS) or administering an intraperitoneal injection of 10 mg/kg body weight LPS. The expression levels of different PKD family isoforms were then detected. Subsequently, experimental groups were established: a control group (CON), an LPS treatment group (LPS), and an LPS plus PKD3 inhibitor group (LPS+CRT). The PKD3-specific inhibitor CRT0066101 was used for intervention under both in vitro and in vivo conditions. The expression levels of pro-inflammatory genes (tnfα, il1β, il8 and il6) and ER stress-related genes (grp78, xbp1, xbp1s, atf4, atf6 and chop) were then measured. The results showed that PKD3 expression was significantly increased in both in vivo and in vitro experiments. Pre-incubation with the PKD3 inhibitor in vitro significantly reduced the LPS-induced increase in the expression of pro-inflammatory genes and ER stress-related genes in large yellow croaker head kidney macrophages. The in vivo experiments also demonstrated that injection of the PKD3 inhibitor significantly attenuated the LPS-induced increase in the expression of these genes in head kidney tissues. These findings indicate that PKD3 plays a crucial role in LPS-induced inflammatory responses and ER stress in large yellow croaker. This study is the first to reveal the key regulatory function of PKD3 in inflammatory responses and ER stress in large yellow croaker. It provides a new molecular basis for understanding the innate immune responses in fish and suggests a potential therapeutic target for managing inflammatory diseases in aquaculture.