The potential of seaweed as a renewable resource is becoming increasingly recognized by diverse stakeholders in Europe. Currently, several initiatives are working on accelerating the development of the European algae industry. Seaweed cultivation can be an important cornerstone in developing EU aquaculture and achieving the European Green Deal. An expert working group was selected and established in February 2021 by the European knowledge brokering mechanism Eklipse. This group was tasked to explore and map the current state of knowledge regarding ecosystem services (ES) provided by seaweed cultivation, including knowledge gaps, constraints, potential negative impacts and tradeoffs. The study was based on the Delphi process and a Quick Scoping Review (QSR). The results of each method showed differences in constraints, negative impacts and knowledge gaps, revealing the need for better communication and collaboration between the involved stakeholders. Both methods identified the following six ES provided by seaweed cultivation: (i) provisioning food, (ii) provisioning hydrocolloids and feed, (iii) regulating water quality, (iv) provisioning habitats, (v) provisioning of nurseries and (vi) regulating climate. Nevertheless, the specific ES identified differed between seaweed taxa. In addition, both methods highlighted also potential negative environmental impacts (e.g., wider ecosystem effects), technological constraints and knowledge gaps (e.g., production). The identified knowledge gaps and constraints were further discussed and prioritized with stakeholders in a workshop in Brussels. This workshop identified the structural research needs for future investigations, including: improved knowledge of environmental impacts; better management of genetic diversity and clear definitions of legal frameworks to support the development of the EU initiative on seaweed sustainable use. This paper summarizes the findings of the investigations of the expert group and future challenges for seaweed cultivation under current and near-future climatic scenarios.