  We report the discovery of the most metal-poor dwarf star-forming galaxy
(SFG) known to date, J0811+4730. This galaxy, at a redshift z=0.04444, has a
Sloan Digital Sky Survey (SDSS) g-band absolute magnitude M_g = -15.41 mag. It
was selected by inspecting the spectroscopic data base in the Data Release 13
(DR13) of the SDSS. LBT/MODS spectroscopic observations reveal its oxygen
abundance to be 12 + log O/H = 6.98 +/- 0.02, the lowest ever observed for a
SFG. J0811+4730 strongly deviates from the main-sequence defined by SFGs in the
emission-line diagnostic diagrams and the metallicity - luminosity diagram.
These differences are caused mainly by the extremely low oxygen abundance in
J0811$+$4730, which is ~10 times lower than that in main-sequence SFGs with
similar luminosities. By fitting the spectral energy distributions of the SDSS
and LBT spectra, we derive a stellar mass of M* = 10^6.24 - 10^6.29 Msun
(statistical uncertainties only), and we find that a considerable fraction of
the galaxy stellar mass was formed during the most recent burst of star
formation.
