The magnetar Swift J1818.0-1607 was discovered in 2020 March 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 wasL(burst) similar to 2 x 10(39)erg s(-1)(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-H= (1.13 0.03) x 10(23)cm(-2)andkT = 1.16 0.03 keV) plus a power law (Gamma = 0.0 1.3) in the 1-20 keV band, with a luminosity of similar to 8 x 10(34)erg s(-1), dominated by the blackbody emission. From our timing analysis, we derive a dipolar magnetic fieldB similar to 7 x 10(14)G, spin-down luminosity (E)over dot(rot) similar to 1.4 x 10(36) erg s(-1), and characteristic age of 240 yr, the shortest currently known. Archival observations led to an upper limit on the quiescent luminosity <5.5 x 10(33)erg s(-1), 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 similar to 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.