An optical fiber humidity sensor was fabricated forming a nanometer-scale Fabry-Perot interferometer by using the Ionic Self-Assembly Monolayer (ISAM) method. The materials used were Poly R-478 and poly(diallyldimethyl ammonium chloride). Taking advantage of the precision that the ISAM method can achieve in controlling the length of the nano cavity, the length was fit to obtain a maximum variation of 8.7 dB of reflected optical power between 11.3% and 85% RH. The sensor exhibited a fast response time and was able to monitor the human breathing.