The remarkable precision of frequency-comb (FC) lasers is transferred to the extreme ultraviolet (XUV, wavelengths shorter than 100nm), a frequency region previously not accessible to these devices. A frequency comb at XUV wavelengths near 51nm is generated by amplification and coherent up-conversion of a pair of pulses originating from a near-infrared femtosecond FC laser. The phase coherence of the source in the XUV is demonstrated using helium atoms as a ruler and phase detector. Signals in the form of stable Ramsey-like fringes with high contrast are observed when the FC laser is scanned over P states of helium, from which the absolute transition frequency in the XUV can be extracted. This procedure yields a He4 ionization energy at h× 5945204212(6)MHz, improved by nearly an order of magnitude in accuracy, thus challenging QED calculations of this two-electron system. © 2010 The American Physical Society.