Extensive time-resolved observations of Kuiper belt object 2001 QG298 show a light curve with a peak-to-peak variation of 1.14+/-0.04mag and single-peaked period of 6.8872+/-0.0002hr. The mean absolute magnitude is 6.85mag, which corresponds to a mean effective radius of 122(77)km if an albedo of 0.04(0.10) is assumed. This is the first known Kuiper belt object and only the third minor planet with a radius greater than 25km to display a light curve with a range in excess of 1mag. We find the colors to be typical for a Kuiper belt object (B-V=1.00+/-0.04, V-R=0.60+/-0.02), with no variation in color between minimum and maximum light.