The newly discovered X-ray transient XTE J1810-197 exhibits almost all characteristics of magnetars. It possesses a relatively long spin period of 5.54 sand a rapid spin down rate of approximate to10(-11) s s(-1), while showing no evidence for Doppler shifts due to a binary companion. This yields a magnetar-strength dipole field B = 3 x 10(14) G and a young characteristic age tau less than or equal to 7600 yr. The spectrum of the source is notably soft (photon index approximate to 4) and optical observations with the 1.5 m Russian-Turkish Optical Telescope RTT150 revealed a limiting magnitude of R-c = 21.5, both consistent with those of Soft gamma repeaters and anomalous X-ray pulsars. However, the source shows a significant flux decline for over nine months and is present in archival ASCA and ROSAT observations at nearly two orders of magnitude fainter luminosity. Putting all evidence together shows that we have found the first confirmed transient magnetar. This suggests the presence of other unidentified transient magnetars in a state similar to XTE J1810-197 in its inactive phase. The detection of such sources is one of the important areas in which future X-ray timing missions can make a significant impact.