Nature-based solutions (NBS) are used to transform existing unsustainable and undesirable path dependencies in cities. For NBS to contribute to just urban transformations, a stronger inter- and transdisciplinary knowledge base is needed. This knowledge base is essential to engage with six complex yet crucial questions about NBS, including “for what?,” “which nature?,” “where?,” “how?,” “when,” and “for whom?.” To address these questions, we identify two critical opportunities to advance the knowledge of NBS. First, we argue for solidifying interdisciplinary approaches to examine how NBS can be designed, planned, and implemented for multifunctionality. Second, we argue that researchers need to work transdisciplinarily with diverse stakeholders to ensure the design, siting, and planning of NBS are appropriate to the context. In both critical opportunities, justice should be a core guiding principle from the beginning of planning the NBS, starting with the foundational understanding that NBS are not inherently just or unjust. Instead, their value depends on a holistic examination of the context in which they operate and the institutional logic that guides their planning. To center justice in the inter- and transdisciplinary research and practice of NBS, a knowledge shift from epistemological injustice to epistemological inclusivity is a critical way forward.