A reconfigurable multiwavelength erbium-doped fiber laser based on an all-fiber multimode interferometer (MMI) is proposed and experimentally demonstrated. The interferometer is constructed by sandwiching a section of highly germanium-doped fiber (HGDF) between two sections of single-mode fiber. The insertion loss of the interferometer is as low as 2 dB. Due to the polarization-dependent spectral filtering effect formed by the MMI, by rotating the intracavity polarization controller, the laser output can be switched among single-, dual-, and triple-wavelength lasing states with optical signal-to-noise ratio up to 50 dB. In particular, the obtained dual-wavelength state shows high stability with wavelength shift within $ \pm {0.04}\;{\rm nm}$±0.04nm, wavelength spacing variation within $ \pm {0.03}\;{\rm nm}$±0.03nm, and power fluctuation within $ \pm {0.04}\;{\rm dB}$±0.04dB by monitoring the output spectra over 8 h at room temperature. By changing the length of the HGDF, the wavelength spacing can also be flexibly manipulated. Taking the advantages of reconfiguration, low cost, and easy fabrication, this fiber laser may have great potential in various optical applications.