We describe lasing of a thulium-doped polarizing photonic crystal fiber. A 4m long fiber with 50 μm diameter core, 250 μm diameter cladding, and d/A ratio of 0.18 was pumped with a 793nm diode and produced a polarized output with a polarization extinction ratio (PER) of 15 dB and an M2 of <1:15. An intracavity polarizer and half-wave plate minimally increased the PER to 16 dB. The output power had 35% slope efficiency relative to the absorbed pump power. The maximum cw output power was limited to 4W due to the quantum defect heating of the fiber.