Research on the link between gender and campaign finance in proportional electoral systems suggests that the campaign expenses of female parliamentary candidates are significantly lower than those of male candidates. On the basis of data on 10,436 candidates for nine consecutive elections in Belgium (1991–2014), this article examines whether there is indeed a gender gap in campaign expenses, and in particular whether this coincides with the introduction of legislative quota laws in the Belgian flexible-list system. We distinguish between realistic candidates that run for election from winnable list positions and unrealistic candidates running from lower ranked positions. The results show that, among unrealistic candidates, the gender gap in campaign spending arose again after the introduction of more severe gender quotas. With regard to realistic list positions, however, the significant difference between male and female candidates in the most strict quota phase disappeared, indicating that female realistic candidates were able to catch up financially with their male counterparts. The Belgian experience could provide useful insights for other countries with flexible-list systems regarding the implementation of legislative gender quotas.