Giant viruses are nonstandard viruses with large particles and genomes. While previous studies have shown that their genomes contain various sequences of interest, their genes related specifically to natural product biosynthesis remain unexplored. Here we analyze the function and structure of a terpene synthase encoded by the gene of a giant virus. The enzyme is phylogenetically separated from the terpene synthases of cellular organisms; however, heterologous gene expression revealed that it still functions as a terpene synthase and produces a cyclic terpene from a farnesyl diphosphate precursor. Crystallographic analysis revealed its protein structure, which is relatively compact but retains essential motifs of the terpene synthases. We thus suggest that like cellular organisms, giant viruses produce and utilize natural products for their ecological strategies.