Seaweed aquaculture accounts for 51.3% of global mariculture production and grows at 6.2% yr−1 (2000–2018). It delivers a broad range of ecosystem services, providing a source of food and natural products across a range of industries. It also offers a versatile, nature-based solution for climate change mitigation and adaptation and for counteracting eutrophication and biodiversity crisis. Here we offer the perspective that scaling up seaweed aquaculture as an emission capture and utilization technology, one supporting a circular bioeconomy, is an imperative to accommodate more than 9 billion people in 2050 while advancing across many of the United Nations Sustainable Development Goals.