The measurement of chloride ion concentrations has been studied for the purpose of monitoring the quality of water resources. In this paper, a chloride ion sensor based on a microfiber coupler with a Sagnac loop is proposed. The microfiber coupler, which acts as the sensing unit and has a diameter of 10 μm and a length of 1 mm, is fabricated using the flame-brushing technique, and the two ends are connected to form a Sagnac loop, which acts as a reflector to enhance the reflection in the structure. Experimental results show that the sensitivity reaches a maximum of 423 pm/‰ and that the detection limit for the chloride ion concentration is 0.447‰ at a wavelength of 1595 nm. The proposed sensor is characterized by a simple and easy manufacturing process, compact structure, and low cost; further, this sensing unit has great potential for applications in marine chloride detection and environmental safety monitoring, especially for monitoring building corrosion and water pollution.