An extremely thermostable magnesium metal–organic framework (Mg-MOF) is reported for use as a highly selective and sensitive, instantaneous and colorimetric sensor for Eu³⁺ ions. There has been extensive interest in the recognition and sensing of ions because of their important roles in biological and environmental systems. However, only a few of these systems have been explored for specific rare earth ion detection. A robust microporous Mg-MOF for the recognition and sensing of Eu³⁺ ions with high selectivity at low concentrations in aqueous solutions has been synthesized. This stable metal–organic framework (MOF) contains nanoscale holes and non-coordinating nitrogen atoms inside the walls of the holes, which makes it a potential host for foreign metal ions. Based on the energy level matching and efficient energy transfer between the host and the guest, the Mg-MOF sensor is both highly selective and sensitive as well as instantaneous; thus, it is a promising approach for the development of luminescent probing materials with unprecedented applications and its use as an Eu³⁺ ion sensor.