We investigate the operational structure of illegal online gambling (IOG) ecosystems by modeling the relationships between gambling sites as multiplex network. We examined the structural properties of the aggregate network as well as each layer within the multiplex network. In general, almost all layers exhibit assortative characteristics, indicating that high-degree nodes tend to connect with nodes of similar attributes. Interlayer and cluster analysis show that the relational layers of illegal online gambling sites exhibit a hierarchically nested structure. The characterization of illegal gambling site networks conducted in this study can provide insights for developing more targeted anti-gambling policies.