Inverse electron demand Diels-Alder cycloadditions are extremely useful tools for orthogonal labeling of biomolecules such as proteins or small molecules in a cellular context. In-cell labeling of dienophile-modified RNA oligonucleotides using Diels-Alder cycloaddition reactions has not been demonstrated before. In this study we report site-specific labeling of RNA oligonucleotides modified with norbornene derivatives at a predefined sequence position within an RNA sequence in vitro and in mammalian cells using various tetrazine-fluorophore conjugates. The approach could in future be used as a chemical tool for the detection and investigation of RNA functions in cells minimizing the presumed distortion of RNA functions by a large chemical reporter group such as a fluorophore.