  We present photometric and spectroscopic observations of SN 2020bio, a
double-peaked Type IIb supernova (SN) discovered within a day of explosion,
primarily obtained by Las Cumbres Observatory and Swift. SN 2020bio displays a
rapid and long-lasting initial decline throughout the first week of its light
curve, similarly to other well-studied Type IIb SNe. This early-time emission
is thought to originate from the cooling of the extended outer hydrogen-rich
(H-rich) envelope of the progenitor star that is shock heated by the SN
explosion. We compare SN 2020bio to a sample of other double-peaked Type IIb
SNe in order to investigate its progenitor properties. Analytical model fits to
the early-time emission give progenitor radius ($\approx$ 100--1500 $R_\odot$)
and H-rich envelope mass ($\approx$ 0.01--0.5 $M_\odot$) estimates that are
consistent with other Type IIb SNe. However, SN 2020bio displays several
peculiarities, including: (1) weak H spectral features indicating a greater
amount of mass loss than other Type IIb progenitors; (2) an underluminous
secondary light-curve peak that implies a small amount of synthesized $^{56}$Ni
($M_{\text{Ni}}$ $\approx$ 0.02 $M_\odot$); and (3) low-luminosity nebular [O
I] and interaction-powered nebular features. These observations are more
consistent with a lower-mass progenitor ($M_{\text{ZAMS}} \approx$ 12
$M_\odot$) that was stripped of most of its H-rich envelope before exploding.
This study adds to the growing diversity in the observed properties of Type IIb
SNe and their progenitors.
