We explore the association between board gender diversity and shareholder value creation. Specifically, we investigate the impact of gender diversity on the economic impact of bank mergers and acquisitions (M&A). We employ a multi-year sample of M&A announced by European listed banks and find that: (i) the presence of women on the board of directors has a positive and statistically significant effect on acquirer gains; and (ii) boards with three or more women, or where women represent more than 25% of the board, have a stronger impact on acquirer gains than in the opposite case, consistent with critical mass theory. Moreover, banks with a critical mass of female directors perform better in undertaking value-enhancing M&A after the global financial crisis. Policy makers and practitioners could benefit from the findings by exploiting the advantages of board heterogeneity in terms of gender.