The optical properties and density of ammonia have not been exhaustively studied despite the relevance of these characteristics in astrophysical scenarios. This work presents new data pertaining to the real part of the refractive index of ammonia at 632.8 nm and the density at different temperatures of deposition ranging from 13 K to ammonia’s desorption temperature of approximately 110 K. The results indicate a significant variability for both parameters versus temperature: an increase of 50% for the density and of 10% for the refractive index as the temperature increases in the range of 13–60 K; at temperatures greater than 60 K, a constant value is reached for both parameters. This initial variation and the following plateau reflect structural differences in the form of ammonia at low and high temperatures of deposition that can play an important role in the interaction of ammonia with other molecules of astrophysical interest.