O-tert-butyl-tyrosine (Tby) is an unnatural amino acid that can be site-specifically incorporated into proteins using established orthogonal aminoacyl-tRNA synthetase/tRNA systems. Here we show that the tert-butyl group presents an outstanding nuclear magnetic resonance (NMR) tag that can readily be observed in one-dimensional 1H NMR spectra without any isotope labeling. Owing to rapid bond rotations and the chemical equivalence of the protons of a solvent-exposed tert-butyl group from Tby, the singlet resonance from the tert-butyl group generates an easily detectable narrow signal in a spectral region with limited overlap with other methyl resonances. The potential of the tert-butyl 1H NMR signal in protein research is illustrated by the observation and assignment of two resonances in the Bacillus stearothermophilus DnaB hexamer (320 kDa), demonstrating that this protein preferentially assumes a three-fold rather than six-fold symmetry in solution, and by the quantitative measurement of the submicromolar dissociation constant Kd (0.2 μM) of the complex between glutamate and the E. coli aspartate/glutamate binding protein (DEBP, 32 kDa). The outstanding signal height of the 1H NMR signal of the Tby tert-butyl group allows Kd measurements using less concentrated protein solutions than usual, providing access to Kd values one order of magnitude lower than established NMR methods that employ direct protein detection for Kd measurements.