  We present six epochs of optical spectropolarimetry of the Type IIP supernova
(SN) 2021yja ranging from $\sim$ 25 to 95 days after the explosion. An
unusually high continuum linear polarization of $p \sim 0.9\%$ is measured
during the early photospheric phase, followed by a steady decrease well before
the onset of the nebular phase. This behavior has not been observed before in
Type IIP supernovae (SNe IIP). The observed continuum polarization angle does
not change significantly during the photospheric phase. We find a pronounced
axis of symmetry in the global ejecta that is shared in common with the
H$\alpha$ and Ca II near-infrared triplet lines. These observations are
consistent with an ellipsoidal geometry. The temporal evolution of the
continuum polarization is also compatible with the SN ejecta interacting with
aspherical circumstellar matter, although no spectroscopic features that may be
associated with strong interaction can be identified. Alternatively, we
consider the source of the high polarization to be an extended hydrogen
envelope that is indistinguishable from low-density circumstellar matter.
