In general, expressing emotions is beneficial and withholding emotions has personal and social costs. Yet, to serve social functions there are situations when emotions are withheld strategically. We examined whether social anxiety influenced when and how emotion expressiveness influences interpersonal closeness in existing romantic relationships. For people with greater social anxiety, withholding the expression of negative emotions was proposed to preserve romantic relationships and their benefits. We examined whether social anxiety and emotion expressiveness interacted to predict prospective changes in romantic relationship closeness over a 12-week period. For people with less social anxiety, relationship closeness was enhanced over time when negative emotions were openly expressed whereas relationship deterioration was found for those more likely to withhold emotions. The reverse pattern was found for people with greater social anxiety such that relationship closeness was enhanced over time for those more likely to withhold negative emotions. Related social anxiety findings were found for discrepancies between desired and actual feelings of closeness over time. Findings were not attributable to depressive symptoms. These results suggest that the costs and benefits of emotion expression are influenced by a person's degree of social anxiety.