We report the presence of an absorption feature in the X-ray spectrum of the BL Lacertae object H1426+428 (z = 0.129). H1426+428 was observed at three separate epochs with BBXRT, ROSAT, and ASCA. In the BBXRT spectrum an absorption feature is clearly present at ~0.66 keV, consistent with absorption by highly ionized oxygen. The phenomenological model that best describes the absorption feature is an inverted Gaussian with width FWHM <~ 43,000 km s-1. This feature is not present in the ROSAT and ASCA data, both taken when the source flux was fainter by a factor ~2. Instead, these data show some evidence for absorption edge(s) at lower energies. These results are consistent with partially ionized, low-density gas surrounding the central engine of H1426+428.As a first approximation the X-ray continuum can be described with a power law with photon index Γ ~ 2. The BBXRT and ASCA data are better fitted with a convex (downward-curved) broken power law. The exact shape of the continuum depends on the assumed profile for the absorption feature. We show that, using a specific warm absorber to fit the data, there is no need for a continuum model more complex than a single power law.