Three-epoch observations of the M1 V–plus–brown dwarf system Gliese 229AB using the Hubble Space Telescope's Wide Field Planetary Camera 2 (WFPC2) are reported. Relative astrometric measurements spanning 1 year confirm the common proper motion of the pair and reveal the first evidence of orbital motion. The radial and azimuthal components of Gl 229B's relative motion are -0087 and +0049, respectively, indicating an elliptical orbit rather than an inclined circular orbit. The absolute WFPC2 magnitudes of Gl 229B are M1042 = 16.37, M814 = 20.76, and M675 = 24.60, assuming a distance of 5.774 pc. The detection of Gl 229B through the F675W bandpass is the first reported at R-band wavelengths. The measured flux through F1042M (λc ≈ 1 μm) is well matched by the latest models of dust-free brown dwarf photospheres by Tsuji et al. These models severely overestimate the broadband fluxes shortward of 0.8 μm, however, which indicates that a strong source of optical continuum opacity exists in the photosphere of Gl 229B. A search for fainter companions to Gl 229A in the Planetary Camera was negative to limiting absolute magnitudes of M1042 ≈ 19 and M814 ≈ 24.5 beyond 7'' of Gl 229A. Nondetection limits for the Wide Field Cameras are 1.5 mag greater than those for the Planetary Camera.
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