The paper analyses with a case study the use of a widely applied normative concept of polycentricity as spatial imaginary. The case study of Helsinki City Plan and the conflict over its city-boulevard scheme draws on qualitative content analysis of planning documents and expert interviews. It demonstrates the instrumental role of multiple interpretations of polycentricity in tension-ridden metropolitan and city-regional spatial planning. The conflict reveals how the conceptual ambiguity of polycentricity and the institutional vagueness of city-regional planning have together enabled advancing contradictory political aims under their guise. In conclusion, the paper emphasizes the persuasive performativity and fluidity of polycentricity as a spatial imaginary in multi-scalar planning settings.