In this Letter, a high-stability, linearly polarized mode-locked polarization-maintaining (PM) Er3+-doped fluoride fiber oscillator at ∼2.8µm is presented for the first time, to the best of our knowledge, where an InAs-based semiconductor saturable absorber mirror is used as the mode locker, and a film polarizer is employed for maintaining the linearly polarized oscillation. In the free-running state, stable linearly polarized mode-locked pulses (τ=44ps and P=446mW) at 2.795 µm, with a high polarization extinction ratio of >23dB and a low integrated relative intensity noise of 0.087% [1 Hz-10 MHz], have been achieved, which can be strongly immune to external mechanical perturbations. By introducing a ruled reflective diffraction grating into the cavity in a Littman configuration, the continuous wavelength tuning of the linearly polarized mode-locked pulses in the range of 2717-2827 nm is obtained as well. To the best of our knowledge, this marks the first demonstration of a linearly polarized PM fiber oscillator in the >2.5µm mid-infrared region.