A waveband-tunable multiwavelength erbium-doped fiber laser based on a novel integrated structure of nonlinear polarization rotation and an unbalanced in-line Sagnac interferometer is proposed and demonstrated. An ultrabroadband lasing spectrum with 140 wavelength lasing peaks periodically distributed from 1563 to 1612.5?nm accompanied by a lasing linewidth of 0.112?nm, a free spectral range (FSR) of 0.32?nm, and a signal-to-noise ratio (SNR) of 20?dB has been achieved. A laser with high-wavelength lasing and waveband tunability was also achieved by merely adjusting the polarization controllers. The wavebands can be tuned from 1567 to 1606?nm continuously. Thirty-one lasing wavelengths in a 3?dB flattened spectrum has been achieved with a SNR of 48?dB, a linewidth of 0.03?nm, and a FSR of 0.32?nm. Good stability of the laser operating at room temperature has been verified by using an optical spectrum analyzer to measure every 2?min for a half-hour.