Filamentous marine cyanobacteria are known to produce a variety of structurally diverse and biologically active secondary metabolites, many of which are being studied for a wide spectrum of applications, ranging from biofuels to pharmaceuticals. The development of a stable heterologous expression host for cyanobacterial biosynthetic pathways would alleviate current bottlenecks in natural products research, such as the low yield of many of these potentially valuable compounds from field-collected extracts. Additionally, it would allow for a deeper investigation of the expression and genetic regulation of these complex pathways while providing a platform in which to investigate pathways from strains for which no genetic tools may yet be developed. To date, there are no known model filamentous cyanobacterial strains of marine origin. In this study we demonstrate the first known transformation and expression of a self-replicating broad-host-range vector and a knockout vector, carrying two reporter genes, in a filamentous marine cyanobacterium.