Guided by a second‐sphere interaction strategy, we fabricated a Tb(III)‐based metal—organic framework (MMCF‐4) for turn‐on sensing of methyl amine with ultra‐low detection limit and high turn‐on efficiency. MMCF‐4 features lanthanide nodes shielded in a nonacoordinate geometry along with secondary coordination spheres that are densely populated with H‐bond interacting sites. Nonradiative routes were inhibited by binding‐induced rigidification of the ligand on the second coordination sphere, resulting in luminescence amplification. Such remote interacting mechanism involved in the turn‐on sensing event was confirmed by single‐crystal X‐ray diffraction and molecular dynamic simulation studies. The design of both primary and secondary coordination spheres of Tb(III) enabled the first turn‐on sensing of organic amines in aqueous conditions. Our work suggests a promising strategy for high‐performance turn‐on sensing for Ln‐MOFs and luminous materials driven by other metal chromophores.