Sub-constituency campaigning occurs when parties focus their campaign resources on specific geographical areas within an electoral district. This behaviour was traditionally thought to occur only in single-member plurality elections, but recent research demonstrates that proportional systems with multi-member districts can also elicit sub-constituency campaigning. However, most studies of sub-constituency campaigning rely on self-reported measures of campaigning, not direct measures of campaign intensity in different regions and communities. We present novel data on geographical variations in the intensity of Flemish parties’ campaign advertising during the 2014 general elections in Belgium, which provides a direct measure of sub-constituency campaigning. Our findings show clear evidence of sub-constituency campaigning: parties campaign more intensely in municipalities where they have stronger electoral support and in municipalities with greater population density.