This article describes a facile fabrication of spinel structured n-type CuAl2O4 thin film with nano-grass like morphological feature by radio frequency (RF) magnetron sputtering technique and the direct realization of this structure as room temperature NH3 gas sensor. X-ray diffraction (XRD) study confirmed the predominant presence of cubic phase CuAl2O4 in the homemade sputtering target and also in the fabricated films. X-ray photoelectron spectroscopic (XPS) investigation confirms the formation of Al3+ and Cu2+ chemical states in the films. Purity and stoichiometry of the deposited films were further confirmed by energy dispersive X-ray spectroscopy (EDX). A randomly aligned nano-grass like morphology with diameters of 25-100 nm and lengths in the range of 0.5 to 2 mu m was observed for the film deposited with 300 W sputtering power on quartz substrate by FESEM study, which provides more reaction sites for gas molecules due to their high aspect ratios (length/diameter). The gas sensing measurements revealed that the prepared n-type CuAl2O4 thin films exhibited superior sensing performance towards ammonia gas in terms of high sensor response, quick response time and recovery time, high sensitivity, long stability, and good reproducibility even at room temperature operation condition. Therefore, we believe that the prepared n-type CuAl2O4 thin films have potential for the detection of ammonia gas leaks in chemical industries.