  We present the discovery and studies of the helium-rich, fast-evolving
supernova (SN) 2021agco at a distance of $\sim$ 40 Mpc. Its early-time flux is
found to rise from half peak to the peak of $-16.06\pm0.42$ mag in the $r$ band
within $2.4^{+1.5}_{-1.0}$ days, and the post-peak light curves also decline at
a much faster pace relative to normal stripped-envelope SNe of Type Ib/Ic. The
early-time spectrum of SN~2021agco ($t \approx 1.0$ days after the peak) is
characterized by a featureless blue continuum superimposed with a weak emission
line of ionized C III, and the subsequent spectra show prominent He I lines.
Both the photometric and spectroscopic evolution shows close resemblances to SN
2019dge, which is believed to have an extremely stripped progenitor. We
reproduce the multicolor light curves of SN 2021agco with a model combining
shock-cooling emission with \Ni decay. The best-fit results give an ejecta mass
of $\approx 0.3$~M$_\odot$ and a synthesized nickel mass of $\approx
2.2\times10^{-2}$~M$_\odot$. The progenitor is estimated to have an envelope
radius $R_{\rm env} \approx 80$~R$_\odot$ and a mass $M_{\rm env} \approx
0.10$~M$_\odot$. All these suggest that SN~2021agco can be categorized as an
ultrastripped SN~Ib, representing the closest object of this rare subtype. This
SN is found to explode in the disk of an Sab-type galaxy with an age of $\sim
10.0$~Gyr and low star-forming activity. Compared to normal SNe Ib/c, the host
galaxies of SN 2021agco and other ultrastripped SNe tend to have relatively
lower metallicity, which complicates the properties of their progenitor
populations.
