Associative issue ownership (AIO) has proven its value in describing issue competition and explaining voting behavior. Yet, it is unclear whether and to what extent AIO also differentiates parties and influences vote choice in highly fragmented, multiparty systems. In such a context, parties must differentiate from many electoral competitors, which makes AIO worth pursuing. At the same time, obtaining unequivocal ownership may be a very difficult endeavor in the face of so many rivals. This paper aims to assess these questions empirically by employing the Dutch Parliamentary Election Study 2021 on a system with 17 elected parties (ENPP = 8). At the aggregate level, we find unequivocal issue ownership for 4 of the 14 issues under study. AIO of most other issues is contested, either by parties with very similar policy positions (within-block competition) or by parties with opposing positions (between-block competition). A final set of issues remain unclaimed. At the individual level, perceptions of issue ownership explain the composition of voters’ party consideration sets (pre-elections) and their actual vote choice (post-elections). These impacts are stronger when voters associate the party with an issue they find important. We conclude that AIO perceptions are an important factor to consider when studying party dynamics and voting behavior in a context of highly fragmented multipartyism.