  We present the discovery of PSO J030947.49+271757.31, the radio brightest
(23.7 mJy at 1.4 GHz) active galactic nucleus (AGN) at z>6.0. It was selected
by cross-matching the NRAO VLA Sky Survey and the Panoramic Survey Telescope
and Rapid Response System PS1 databases and its high-z nature was confirmed by
a dedicated spectroscopic observation at the Large Binocular Telescope. A
pointed Neil Gehrels $Swift$ Observatory XRT observation allowed us to measure
a flux of $\sim$3.4$\times$10$^{-14}$ erg s$^{-1}$ cm$^{-2}$ in the [0.5-10]
keV energy band, which also makes this object the X-ray brightest AGN ever
observed at z>6.0. Its flat radio spectrum ($\alpha_{\nu r}$<0.5), very high
radio loudness (R>10$^3$), and strong X-ray emission, compared to the optical,
support the hypothesis of the blazar nature of this source. %i.e. a radio-loud
(RL) AGN with the relativistic jet pointed toward us. Assuming that this is the
only blazar at this redshift in the surveyed area of sky, we derive a space
density of blazars at z$\sim$6 and with M$_{1450 \mbox{\AA}}$ < -25.1 of
5.5$^{+11.2}_{-4.6}$$\times$10$^{-3}$ Gpc$^{-3}$. From this number, and
assuming a reasonable value of the bulk velocity of the jet ($\Gamma$=10), we
can also infer a space density of the entire radio-loud AGN population at
z$\sim$6 with the same optical/UV absolute magnitude of 1.10$^{+2.53}_{-0.91}$
Gpc$^{-3}$. Larger samples of blazars will be necessary to better constrain
these estimates.
