Marine sponges are distributed in the water, from the intertidal zones to thousands of meters deep. They are primitive multicellular invertebrates that live in benthic environments and are bound to solid substrates. Filter feeders, sponges have many microscopic pores on their surface, which allow water to enter and circulate via a network of canals where microbes and organic particles are filtered out and absorbed. Marine fungi are widespread in the oceans and colonize different ecological niches; they are found associated with organisms of all trophic levels and can act as saprobes, symbionts, and parasites. Compared with other marine microorganisms, marine fungus is relatively understudied. Fungi associated with sponges have been discovered to be a promising source of pharmacologically active compounds with unique anticancer, antibacterial, and antiviral properties.