Microporous cellulose xerogel can be defined as low density biomaterial that can be employed for a variety of promising applications of different fields. The characteristics of xerogel are a consequence of their microstructure. An easy-to-use and reversible solid-state colorimetric sensor for ammonia gas was developed by embedding a bromocresol purple (BCP) pH-sensory chromophore into the environmental friendly carboxymethyl cellulose as bio-based polymer (CMC) matrix. The bromocresol purple was immobilized into cross-linked carboxymethyl cellulose (CMC-BCP) xerogel followed by freeze-drying to introduce a microporous network of regenerated cellulose host in which bromocresol purple chromophore was immobilized to function as a spectroscopic probe guest. Identification of ammonia gas occurred via proton shift from the hydroxyl group of the BCP dye to ammonia nitrogen. Both qualitative and quantitative activities were determined. The architectures of the prepared cellulose xerogel at different degree of substitutions (DS) was investigated using Fourier-transform infrared spectroscopy (FTIR) and scan electron microscopy (SEM), which displayed a high porosity and pores diameter in the range of 10-50 μm. The resultant CMC-BCP displayed high sensitivity for gaseous ammonia. Moreover, excellent reversibility and short detection time were also monitored. The vapochromic xerogel provided an instant color alteration signal from yellow to purple when exposed to ammonia gas or an ammonium hydroxide aqueous environment as monitored by the absorption maxima, color coordinates and color strength. The visual color change of CMC-BCP xerogel was observed to alter in the order from yellow, orange, red to purple in proportional with raising the ammonia concentration in an aqueous environment. Moreover, the CMC-BCP xerogel displayed rapid response time, concentration detection limit as low as 9.0 × 10 ppb for ammonia in aqueous media, and very good reversibility.