Union density has fallen dramatically in many western European countries over recent decades, with grave implications for unions' strength and legitimacy regarding collective bargaining. Free‐riding remains an ongoing challenge in maintaining union membership levels, as covered nonunion workers generally receive the same collectively bargained terms as union members. To help resolve this problem, we propose a union default that would, as a legal requirement, automatically enrol all workers under coverage as union members in the first instance, but with a subsequent right to opt‐out. We use survey evidence from New Zealand to suggest that nearly half these nonunion workers working for unionised employers would retain membership if defaulted. A default would be effective at converting nonunion workers into union members partly because it would shift the norms of the workplace from nonunion to union, as per social custom theory.