  We present high-cadence optical, ultraviolet (UV), and near-infrared data of
the nearby ($D\approx23$ Mpc) Type II supernova (SN) 2021yja. Many Type II SNe
show signs of interaction with circumstellar material (CSM) during the first
few days after explosion, implying that their red supergiant (RSG) progenitors
experience episodic or eruptive mass loss. However, because it is difficult to
discover SNe early, the diversity of CSM configurations in RSGs has not been
fully mapped. SN 2021yja, first detected within ${\approx}5.4$ hours of
explosion, shows some signatures of CSM interaction (high UV luminosity, radio
and x-ray emission) but without the narrow emission lines or early light curve
peak that can accompany CSM. Here we analyze the densely sampled early light
curve and spectral series of this nearby SN to infer the properties of its
progenitor and CSM. We find that the most likely progenitor was an RSG with an
extended envelope, encompassed by low-density CSM. We also present archival
Hubble Space Telescope imaging of the host galaxy of SN 2021yja, which allows
us to place a stringent upper limit of ${\lesssim}9\ M_\odot$ on the progenitor
mass. However, this is in tension with some aspects of the SN evolution, which
point to a more massive progenitor. Our analysis highlights the need to
consider progenitor structure when making inferences about CSM properties, and
that a comprehensive view of CSM tracers should be made to give a fuller view
of the last years of RSG evolution.

High Energy Astrophysical Phenomena;Solar and Stellar Astrophysics