Polychaetes (such as Sternaspis sculata) can biotransform polycyclic aromatic hydrocarbons (PAHs) in the environment, and the cytochrome P450 (CYP) enzyme is the key substance in this process. Here, a novel CYP gene was identified from S. sculata, a marine polychaete that is abundantly distributed along marine coasts worldwide. The full-length cDNA of the new CYP is a 1829 bp, encoding a protein of 509 amino acids. The deduced amino acid sequence of the CYP gene contains the conserved motifs of the P450 family (FxxGxxxCxG) and the characteristic sequences of the CYP4 family (EVDTFMFEGHDTT), showing high similarity to CYP4V from marine polychaetes. Therefore, the cDNA sequence might belong to the subfamily of CYP4V, tentatively named SsCYP4V. The expression levels of SsCYP4V in the field were detected in S. sculata sampled from six sites along the coasts of the Liaodong Peninsula. The results showed that the relative expression levels of SsCYP4V in S. sculata were significantly different among the sampling sites; however, no positive relationship was found between SsCYP4V expression levels and PAHs concentrations in the sediments. These findings suggested a complexity of CYP gene expression in the field, and SsCYP4V cannot be used as a biomarker for the field monitoring of PAH pollution.