The Ultraviolet Spectrum and Continuum Energy Distribution of the Bright Quasar H1821+643

We report on the first UV observations of the bright QSO H1821+643. With V = 14.2 mag and z = 0.297, H1821+643 is the second brightest object in the sky at z > 0.1. The IUE data are combined with new optical spectroscopy, and existing infrared and X-ray data, to reveal a strong optical/UV "big bump," which continues past the Lyman limit in the rest frame of the QSO. A possible turnover at the high- frequency side of the UV continuum constrains fits of a thin accretion disk model to a large black hole mass (M ~3 x 10^9^ M_sun_) and high accretion rate (M ~19 M_sun_ yr^-1^), but a small disk size (R_out_/R_in_~12). The shape of the UV continuum was found to be variable, with a hardening of the spectrum when the source was brighter. Because of its location only 3^deg^ from the ecliptic pole, H1821+643 will be an important object for simultaneous UV and soft X- ray monitoring to test for a common origin of the UV bump and soft X- ray excess.