We have mapped the polarization of the 100 μm thermal emission from the molecular cloud M17. We use the results to infer characteristics of the magnetic field in the cloud and to infer general relationships between the polarized emission and the environment of the emitting grains. Overall, the field configuration appears quite orderly. The field lines are drawn into the cloud core and are deflected away from the H II region. The degree of polarization decreases with increasing optical depth. No correlation is found between the degree of polarization and the dust temperature.