A great deal of evidence has recently been gathered in favor of the picture that Soft Gamma Repeaters and Anomalous X-Ray Pulsars are powered by ultra-strong magnetic fields (B > 10^{14} G; i.e. magnetars). Nevertheless, present determination of the magnetic field in such magnetar candidates has been indirect and model dependent. A key prediction concerning magnetars is the detection of ion cyclotron resonance features, which would offer a decisive diagnostic of the field strength. Here we present the detection of a 5 keV absorption feature in a variety of bursts from the Soft Gamma Repeater SGR 1806-20, confirming our initial discovery (Ibrahim et al. 2002) and establishing the presence of the feature in the sources burst spectra. The line feature is well explained as proton cyclotron resonance in an ultra-strong magnetic field, offering a direct measurement of SGR 1806-20s magnetic field (B ~ 10^{15} G) and a clear evidence of a magnetar. Together with the sources spin-down rate, the feature also provides the first measurement of the gravitational redshift, mass and radius of a magnetar.
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