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There is growing demand for convenient and cost-efficient ethanol sensing techniques in daily life and industry. However, the complexity, high cost, and large volume of the equipment hinder the widespread use of conventional ethanol sensing techniques. Here, we introduce a test paperlike visualized label-free ethanol sensing platform by taking advantage of the instability of 2D Ruddlesden–Popper perovskite in ethanol. The photoluminescence of PEA2Cs n–1Pb n Br3n+1 2D perovskite changes from deep blue to green distinctly when exposed to ethanol. A cheap demo ethanol test paper is fabricated by this 2D perovskite and paper using a simple method, demonstrating interesting rapid and convenient ethanol sensing performance. Crystal analysis, constituent analysis, spectral analysis, and molecular dynamic (MD) simulation reveal the crystal structure transformation from 2D to 3D induced by the migration of PEA+ cations in the original 2D perovskite when exposed to ethanol. In addition, a chemiresistive sensor based on the 2D perovskite and on the 2D–3D phase transformation is developed and demonstrates a high ethanol detection sensitivity with a limit down to 0.7 ppm. The photodetector fabricated by the 2D perovskite also demonstrates tunable sensitivity upon the controllable 2D–3D transformation. This remarkable colorimetric, chemiresistive, and photosensitive perovskite is expected to highlight its future application and be a supplement to the existing ethanol sensing and photodetector techniques.