Real-time measurement of intracellular pH in live cells is of great importance for understanding physiological/pathological processes and developing intracellular drug delivery systems. We report here the first use of nanoscale metal-organic frameworks (NMOFs) for intracellular pH sensing in live cells. Fluorescein isothiocyanate (FITC) was covalently conjugated to a UiO NMOF to afford F-UiO NMOFs with exceptionally high FITC loadings, efficient fluorescence, and excellent ratiometric pH-sensing properties. Upon rapid and efficient endocytosis, F-UiO remained structurally intact inside endosomes. Live cell imaging studies revealed endo- and exocytosis of F-UiO and endosome acidification in real time. Fluorescently labeled NMOFs thus represent a new class of nanosensors for intracellular pH sensing and provide an excellent tool for studying NMOF-cell interactions.