We demonstrate a novel surround modulation of global form perception by using Glass patterns in a center-surround configuration. Glass patterns contain randomly distributed dot pairs, or dipoles, whose orientations are determined by a geometric transform. By integrating across dipoles, an observer can perceive a global structure in the image. We measured the coherence threshold, the minimum proportion of signal dots needed for an observer to detect the global form, at 75% accuracy. The coherence thresholds of the central target Glass patterns were measured either alone or with the presence of various Glass pattern surrounds. Concentric and spiral surrounds increased the coherence threshold for the concentric target compared with that measured for the target alone, while a radial surround had no effect. The coherence threshold for the radial pattern was elevated only by the spiral surround while the spiral and translational targets were not affected by any surrounds. The effect persisted when the center and the surround were segregated by a blank gap and peaked at an optimal gap width. Our results show that global form perception can be modulated by a surround, and this modulation depends on the shapes of the central target and the surround context.