In this work, a new post‐synthetic hydrothermal approach leading to simultaneous dealumination and silylation of zeolite nanocrystals is presented. Starting from a nanosized chabazite (CHA) type zeolite with a Si/Al ratio equal to 2, a set of samples with Si/Al ratios up to 4 are obtained by altering the acidity and concentration of the added silica modifier during the post‐synthesis treatment leading to concurrent dealumination and silylation. The effect of the acidity during the post‐synthetic silylation treatment at different pH of 1.0–8.5 at a constant silicon concentration is investigated. The nanosized CHA samples are characterized in details by Fourier transform infrared spectroscopy and NMR spectroscopy (1H, 29Si, 27Al), X‐ray diffraction, and thermogravimetric analyses. The post‐synthesis approach reported here can be applied to other zeolite types for fine‐tuning of their structural features including type and location of T‐atoms, silanol sites, and active sites.