Cyanophages are viruses that infect cyanobacteria. Cyanophages can be found in both freshwater and marine environments. Marine and freshwater cyanophages have icosahedral heads, which contain double-stranded DNA, attached to a tail by connector proteins. The size of the head and tail vary among species of cyanophages. 
Cyanophages, like other bacteriophages, rely on Brownian motion to collide with bacteria, and then use receptor binding proteins to recognize cell surface proteins, which leads to adherence. Viruses with contractile tails then rely on receptors found on their tails to recognize highly conserved proteins on the surface of the host cell.
Cyanophages infect a wide range of cyanobacteria and are key regulators of the cyanobacterial populations in aquatic environments, and may aid in the prevention of cyanobacterial blooms in freshwater and marine ecosystems. These blooms can pose a danger to humans and other animals, particularly in eutrophic freshwater lakes. Infection by these viruses is highly prevalent in cells belonging to Synechococcus spp. in marine environments, where up to 5% of cells belonging to marine cyanobacterial cells have been reported to contain mature phage particles.
The first cyanophage, LPP-1, was discovered in 1963. Cyanophages are classified within the bacteriophage families Myoviridae (e.g. AS-1, N-1), Podoviridae (e.g. LPP-1) and Siphoviridae (e.g. S-1).