  The magnetar Swift ,J1818.0-1607 was discovered in March 2020 when Swift
detected a 9 ms hard X-ray burst and a long-lived outburst. Prompt X-ray
observations revealed a spin period of 1.36 s, soon confirmed by the discovery
of radio pulsations. We report here on the analysis of the Swift burst and
follow-up X-ray and radio observations. The burst average luminosity was
$L_{\rm burst} \sim2\times 10^{39}$ erg/s (at 4.8 kpc). Simultaneous
observations with XMM-Newton and NuSTAR three days after the burst provided a
source spectrum well fit by an absorbed blackbody ($N_{\rm H} = (1.13\pm0.03)
\times 10^{23}$ cm$^{-2}$ and $kT = 1.16\pm0.03$ keV) plus a power-law
($\Gamma=0.0\pm1.3$) in the 1-20 keV band, with a luminosity of
$\sim$$8\times10^{34}$ erg/s, dominated by the blackbody emission. From our
timing analysis, we derive a dipolar magnetic field $B \sim 7\times10^{14}$ G,
spin-down luminosity $\dot{E}_{\rm rot} \sim 1.4\times10^{36}$ erg/s and
characteristic age of 240 yr, the shortest currently known. Archival
observations led to an upper limit on the quiescent luminosity
$<$$5.5\times10^{33}$ erg/s, lower than the value expected from magnetar
cooling models at the source characteristic age. A 1 hr radio observation with
the Sardinia Radio Telescope taken about 1 week after the X-ray burst detected
a number of strong and short radio pulses at 1.5 GHz, in addition to regular
pulsed emission; they were emitted at an average rate 0.9 min$^{-1}$ and
accounted for $\sim$50% of the total pulsed radio fluence. We conclude that
Swift ,J1818.0-1607 is a peculiar magnetar belonging to the small, diverse
group of young neutron stars with properties straddling those of rotationally
and magnetically powered pulsars. Future observations will make a better
estimation of the age possible by measuring the spin-down rate in quiescence.
