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We present the discovery and studies of the helium-rich, fast-evolvingsupernova (SN) 2021agco at a distance of $\sim$ 40 Mpc. Its early-time flux isfound to rise from half peak to the peak of $-16.06\pm0.42$ mag in the $r$ bandwithin $2.4^{+1.5}_{-1.0}$ days, and the post-peak light curves also decline ata much faster pace relative to normal stripped-envelope SNe of Type Ib/Ic. Theearly-time spectrum of SN~2021agco ($t \approx 1.0$ days after the peak) ischaracterized by a featureless blue continuum superimposed with a weak emissionline of ionized C III, and the subsequent spectra show prominent He I lines.Both the photometric and spectroscopic evolution shows close resemblances to SN2019dge, which is believed to have an extremely stripped progenitor. Wereproduce the multicolor light curves of SN 2021agco with a model combiningshock-cooling emission with \Ni decay. The best-fit results give an ejecta massof $\approx 0.3$~M$_\odot$ and a synthesized nickel mass of $\approx2.2\times10^{-2}$~M$_\odot$. The progenitor is estimated to have an enveloperadius $R_{\rm env} \approx 80$~R$_\odot$ and a mass $M_{\rm env} \approx0.10$~M$_\odot$. All these suggest that SN~2021agco can be categorized as anultrastripped SN~Ib, representing the closest object of this rare subtype. ThisSN is found to explode in the disk of an Sab-type galaxy with an age of $\sim10.0$~Gyr and low star-forming activity. Compared to normal SNe Ib/c, the hostgalaxies of SN 2021agco and other ultrastripped SNe tend to have relativelylower metallicity, which complicates the properties of their progenitorpopulations.

Discovery of the Closest Ultrastripped Supernova: SN 2021agco in UGC 3855