Ammonia has been an important industrial colorless agent. Exposure to gaseous ammonia results in organ damage or even death. Herein, an environmentally friendly colorimetric detector for aqueous and gaseous ammonia was prepared utilizing vapochromic polylactic acid nanofibers. Betalain (BTN) has been reported as a natural probe that can be extracted from the beetroot plant (Beta vulgaris L.). Mordant (M)/BTN coordinating complex nanoparticles were produced in situ by depositing the Betalain probe onto polylactic acid (PLA) nanofibers. The colorimetric change of the Betalain-dyed PLA nanofibers from red to yellow when exposed to ammonia was examined using both absorbance spectra and coloration parameters. The PLA membrane displayed a detection limit of 5-400 ppm. Upon exposure to ammonia, the absorbance spectra of the nanofibrous membrane showed a hypsochromic shift, moving from 572 nm to 402 nm with an isosbestic wavelength of 466 nm. Scanning electron microscopy (SEM) analysis demonstrated that the nanofibrous membrane had diameters of 100-350 nm. Transmission electron microscopy (TEM) analysis of the M/BTN particles revealed diameters of 10-13 nm. After immobilizing the M/BTN nanoparticles onto the nanofibrous membrane, no substantial variations in the bend length and air permeability were observed. The colorfastness of the Betalain-dyed nanofibrous membrane showed satisfactory results.