Green fluorescent protein (GFP) is a protein that emits green fluorescence when exposed to a radiation of ultraviolet wavelength range, even without the addition of substrate and cofactor. Because of such characteristics, the usage of GFP is widespread in both in vivo and in vitro applications. In addition, recent advances in biotechnology have enabled GFP to be expressed in various hosts, including bacteria, yeast, plants, animals, and even living-cells, for multiple purposes. Currently, GFP is a subject of great interest in the analytical sciences, especially in immunoassays for qualitative and quantitative analyses, when it is fused with an antibody because of the high sensitivity of GFP and antigen-binding specificity of antibodies. Recently the fluobody, which is a fusion protein of GFP with single-chain variable fragment antibody (scFv), has become a useful tool in various fields. We review here the applications of GFP as fluobodies in immunoassays.