Luminescence detection is a sensitive approach for high-resolution visualization of nano-/macrosized objects, but it is challenging to light invisible insulators owing to their inert surfaces. Herein, we discovered a steric restriction-induced emission (SRIE) effect on nanoscale insulators to light them by fluorogenic probes. The SRIE effect enabled us to specifically differentiate a representative nanoscale insulator, boron nitride (BN) nanosheets, from 18 tested nanomaterials with 420-fold increments of photoluminescence intensity and displayed 3 orders of magnitude linearity for quantitative analysis as well as single-particle level detection. Molecular dynamics simulations indicated that the hydrophobic and electron-resistant surfaces of BN nanosheets restricted intramolecular motions of fluorogenic molecules for blockage of the nonradiative path of excited electrons and activation of the radiative electron transition. Moreover, the lighted BN nanosheets could be successfully visualized in complex cellular and tissue biocontexts. Overall, the SRIE effect will inspire more analytical techniques for inert materials.