Extracellular signal-regulated kinases (ERKs) are conserved and related with protein-serine/threonine kinases that participate in the regulation of multiple biological processes, such as cell survival, cell differentiation, proliferation, metabolism, and inflammation. However, little is known about the roles of this kinase in the pearl oyster. In this study, we cloned and identified an ERK homolog from Pinctada fucata (PfErk). Furthermore, we have unraveled its expressional kinetics after lipopolysaccharide (LPS) and polyinosinic-epolycytidylic acid (poly I:C) immune challenge. Pferk harbored a 5′ untranslated region (UTR) of 12 bp, a coding sequence of 1074 bp, and a 3′ UTR of 882 bp. The putative peptide comprised a predicted molecular mass of 41.19 kDa, with a theoretical pI of 6.15. Sequence analysis showed that it possesses one STK catalytic domain and a conserved His-Arg-Asp (HRD) domain. In addition, a canonical Thr-Glu-Tyr (TEY) dual phosphorylation motif and an ATRW substrate binding site were also identified in the coding protein. Homology assessment of PfErk showed high similarity to Homo sapiens ERK. Phylogenetic analysis supported a close evolutionary relationship with molluscan orthologs. The expression patterns of Pferk were observed in seven different tissues of pearl oyster, with highest expression in the mantle and lowest expression in the digestive gland. Pferk mRNA expression levels were detected at developmental stages, with the highest expression in D-shaped larvae, followed by the 32-cell stage. The mRNA expression of Pferk was upregulated significantly in P. fucata mantle primary cells and mantle tissue after LPS and poly (I:C) treatment, and PfErk phosphorylation levels were activated by LPS and poly (I:C) challenges. Overall, our results suggested that PfErk may play important roles in pearl oyster innate immunity, and provided a new understanding of mantle immunity in the pearl oyster.