  We present three epochs of early-time ultraviolet (UV) and optical HST/STIS
spectroscopy of the young, nearby Type IIP supernova (SN) 2021yja. We
complement the HST data with two earlier epochs of Swift UVOT spectroscopy. The
HST and Swift UVOT spectra are consistent with those of other well-studied Type
IIP supernovae (SNe). The UV spectra exhibit rapid cooling at early times,
while less dramatic changes are seen in the optical. We also present Lick/KAIT
optical photometry up to the late-time-tail phase, showing a very long plateau
and shallow decline compared with other SNe IIP. Our modeling of the UV
spectrum with the TARDIS radiative-transfer code produces a good fit for a
high-velocity explosion, a low total extinction $E(B-V) = 0.07$ mag, and a
subsolar metallicity. We do not find a significant contribution to the UV flux
from an additional heating source, such as interaction with the circumstellar
medium, consistent with the observed flat plateau. Furthermore, the velocity
width of the Mg II $\lambda$2798 line is comparable to that of the hydrogen
Balmer lines, suggesting that the UV emission is confined to a region close to
the photosphere.
