We present polarization spectra near maximum light for the strongly subluminous Type Ia supernova 1999by that show that the supernova is intrinsically polarized. SN 1999by has an observed, overall level of polarization of ~0.3 to 0.8%, a rise of the polarization P redward of 6500 A, and a change in polarization across the Si II 6150 A feature of about 0.4%. The distribution of points with wavelength using an empirical Q-U plane method reveals that SN 1999by has a well-defined axis of symmetry and suggests an interstellar polarization (ISP) vector with P(ISP)=0.3% and position angle Theta = 150 deg with an error circle in the Q-U plane of radius about 0.1%. Synthetic NLTE-spectra for axisymmetric configurations based on delayed detonation models have been computed assuming ellipsoidal geometry. Both flux and polarization spectra can be reasonably well reproduced by models with an asphericity of ~20 % observed equator-on. The general properties of the polarization can be understood as a consequence of the structure of subluminous models. The low upper limits for polarization determined for many normal events in contrast to the high polarization in SN 1999by may suggest a relation between the asymmetry we observed and the mechanism that produces a subluminous Type Ia. Among various mechanisms, rapid rotation of the progenitor white dwarf, or an explosion during a binary white dwarf merger process are likely candidates to explain the asphericity in SN 1999by.
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