  We report spectropolarimetric observations of the Type Ia supernova (SN)
2021rhu at four epochs: $-$7, +0, +36, and +79 days relative to its $B$-band
maximum luminosity. A wavelength-dependent continuum polarization peaking at
$3890 \pm 93$ Angstroms and reaching a level of $p_{\rm max}=1.78% \pm 0.02$%
was found. The peak of the polarization curve is bluer than is typical in the
Milky Way, indicating a larger proportion of small dust grains along the
sightline to the SN. After removing the interstellar polarization, we found a
pronounced increase of the polarization in the CaII near-infrared triplet, from
$\sim$0.3% at day $-$7 to $\sim$2.5% at day +79. No temporal evolution in
high-resolution flux spectra across the NaID and CaIIH&K features was seen from
days +39 to +74, indicating that the late-time increase in polarization is
intrinsic to the SN as opposed to being caused by scattering of SN photons in
circumstellar or interstellar matter. We suggest that an explanation for the
late-time rise of the CaII near-infrared triplet polarization may be the
alignment of calcium atoms in a weak magnetic field through optical
excitation/pumping by anisotropic radiation from the SN.
