Mathematical expertise builds on a foundation of space—especially the ability to map exact numbers to linear space. This “mental number-line” is known to vary cross-culturally, but there is debate about the mechanisms responsible for its cultural elaboration. We investigated the role of co-speech gesture, a ubiquitous cultural activity, in stabilizing and entrenching the mental number-line within a community. Imitating culture-specific gestures systematically shaped gesturers’ mental number-line. Moreover, gestures were used spontaneously to infer speakers’ spatial understanding of number, and merely observing these gestures was sufficient to shape the observer’s own mental number-line. These findings establish co-speech gesture as one mechanism for propagating and perpetuating the number-line.