It is shown that a sine-Gordon equation with additive white noise, formally given by
on an open bounded with smooth boundary, where , , is the formal derivative of a one-dimensional Wiener process, imposing Dirichlet conditions, has a random attractor. A nonrandom upper bound for the Hausdorff dimension of the random attractor, which decreases as the damping grows, is derived. The Hausdorff dimension of random attractors has been shown to be nonrandom almost surely by H. Crauel and F. Flandoli [J. Dyn. Differ. Equations 10, 449-474 (1998; Zbl 0927.37031)].