The detection of ultralow heavy metal ion concentration is highly significant for protecting human health and maintaining the stability of the ecological environment. Herein, a microfiber interferometer chemical sensor for the detection of Ni2+ ions was proposed and experimentally demonstrated. The microfiber sensor was coated with an ion-imprinted chitosan polymer using Ni2+ as the template ion. Experimental results demonstrated a high sensitivity of 0.0454 nm nM−M for detect-ing Ni2+ in the range of 10 nM to 100 nM, and a limit of detection as low as 6.5 nM was achieved. The microfiber sensor was verified using two different non-template heavy ions, Cu2+ and Cr3+, and was determined to be highly selective to Ni2+. Furthermore, the regeneration characteristics of the sensor were experimentally assessed by three repeated adsorption–desorption cycles, and the results showed that the microfiber sensor achieved good stability without a significant loss in sensitivity. Besides, the detecting tests of Ni2+ in lake water and industrial sewage samples demonstrated the sensor's practical application. This proposed sensor has the advantages of simple configuration, high selectivity and sensitivity, fast response, and the ability to serve as a platform for water safety monitoring and remote sensing.