Fluorescent and bioluminescent genetically encoded Ca indicators (GECIs) are an indispensable tool for monitoring Ca dynamics in numerous cellular events. Although fluorescent GECIs have a high spatiotemporal resolution, their application is often confined to short-term imaging due to the external illumination that causes phototoxicity and autofluorescence from specimens. Bioluminescent GECIs overcome these pitfalls with enhanced compatibility to optogenetic manipulation and photophysiological processes; however, they are compromised for spatiotemporal resolution. Therefore, there has been a push toward the use of Ca indicators that possess the advantages of both fluorescent and bioluminescent GECI for a wide range of applications. To address this, we developed a high-affinity bimodal GECI, GLICO, using a single fluorescent protein-based GECI combined with a split luciferase. Through this novel design, the fusion protein becomes bimodal and possesses Ca sensing properties similar to those of its fluorescent ancestor and confers bioluminescence-based Ca imaging. GLICO in bioluminescence mode has the highest dynamic range (2200%) of all bioluminescent GECIs. We demonstrated the performance of GLICO in studying cytosolic Ca dynamics in different cultured cells in each mode. With the purpose of Ca imaging in high Ca content organelle, we also created a low-affinity variant, ReBLICO and performed Ca imaging of the ER in both fluorescence and bioluminescence modes. The ability to switch between fluorescence and bioluminescence modes with a single indicator would benefit transgenic applications by presenting an opportunity for a wide range of live Ca imaging in physiological and pathophysiological conditions.