We developed a new fluorogenic bioorthogonal reaction that is based on the inverse electron-demand Diels-Alder reaction between styrene (an unstrained alkene) and a simple tetrazine. The reaction forms a new fluorophore with no literature precedent. We have identified an aminoacyl-tRNA synthetase/tRNA pair for the efficient and site-specific incorporation of a styrene-containing amino acid into proteins in response to amber nonsense codon. Fluorogenic labeling of purified proteins and intact proteins in live cells were demonstrated. The fluorogenicity of the styrene-tetrazine reaction can be potentially applied to the study of protein folding and function under physiological conditions with low background fluorescence interference.