Cyanobacteria are photosynthetic prokaryotes, capable of sustaining their growth by converting sunlight into chemical energy by fixing CO2 into organic matter. The cyanobacterium Anabaena sp. PCC 7120 is also capable of fixing atmospheric nitrogen, a metabolic process that occurs in specialized cells, the heterocysts. During the process of heterocyst differentiation, drastic morphological changes occur to prepare the future differentiated cell to accommodate the nitrogen fixation metabolism, which is a highly O2-sensitive process. Recently, we identified an unknown extracellular protein (termed HesF) in Anabaena sp. PCC 7120 and found it to be required for the proper deposition of the polysaccharide layers in the heterocyst cell wall. HesF is a non-classical type I secretion system (T1SS)-dependent secreted substrate, and its secretion signal remained elusive. Here, we report that the secretion signal of HesF is located in its C-terminus. We present evidence that a heterologous reporter protein fused with HesF's secretion signal could be successfully expressed in heterocysts and secreted to the extracellular medium, following hesF's native regulation. This represents the first time that the secretion signal of a cyanobacterial T1SS-dependent substrate is identified, and demonstrates the feasibility of using cyanobacteria for selected protein expression and secretion.