As of 2021, atmospheric carbon dioxide levels have reached historically unprecedented levels, higher than at any time in the past 800,000 years. Worldwide efforts to reduce emissions by creating a more efficient, carbon-free energy system may not be enough to stabilize the climate and avoid the worst impacts of climate change. Carbon dioxide removal (CDR) strategies, which remove and sequester carbon from the atmosphere, likely will be needed to meet global climate goals. The ocean, covering 70% of the Earth's surface, includes much of the global capacity for natural carbon sequestration; the ocean also holds great potential for uptake and longerterm sequestration of human-produced CO2. This report builds on previous work from the National Academies to assess what is currently known about the benefits, risks, and potential for responsible scale-up of six specific ocean-based CDR strategies as identified by the sponsor, ClimateWorks Foundation. It describes the research needed to advance understanding of those approaches and address knowledge gaps. The resulting research agenda is meant to provide an improved and unbiased knowledge base for the public, stakeholders, and policymakers to make informed decisions on the next steps for ocean CDR, as part of a larger climate mitigation strategy; it is not meant to lock in or advocate for any particular approach.