This paper describes the design and operation of a stable single-longitudinal-mode medium-power multiwavelength fiber ring laser at room temperature. Single-longitudinal-mode oscillation was achieved by using an unpumped polarization-maintaining erbium-doped fiber inside the cavity as a saturable absorber. To investigate the function of the saturable absorber, erbium-doped fibers with different erbium concentrations and lengths were used. The output characteristics of the laser were studied using a scanning Fabry-Perot spectrum analyzer of resolution 27 MHz and an optical spectrum analyzer of resolution 6.25 GHz. The total output power of the laser obtained was more than 100 mW. The minimum separation between lasing wavelengths for stable single-longitudinal-mode oscillations, number of lasing lines, tunability, and maximum available output power were also explored.