Abstract Bright, sensitive fluorescent probes that respond to changes in the cellular microenvironment are extremely valuable for imaging cellular dynamics. We report a simple, one‐step synthesis of a new hemicaynine (HC‐1) dye as a sensitive, red‐emitting (λmax‐610 nm) fluorogenic probe for micro‐viscosity and local order in diverse environments, including live cells. HC‐1 responds to increasing micro‐viscosity through changes in fluorescence intensity and lifetime, and is sensitive enough to report dynamic micellar self‐assembly. While HC‐1 shows properties of a molecular ‘rotor’, time‐dependent density functional theoretical analysis reveals that in HC‐1, an inhibition of photo‐isomerization in viscous environment is the likely cause of fluorescence enhancement. HC‐1 localizes to mitochondria in live cells and responds to mitochondrial ordering through a significant increase in fluorescence. Strikingly, we show that HC‐1 is also a sensitive probe for the spatial heterogeneity of mitochondrial organization in embryonic stem cells as well as dynamic remodeling of the mitochondria in early‐differentiated cells.