Biomass-based marine CO2 removal (mCDR) aims to harness photosynthetic organisms to remove excess CO2 from the atmosphere and sequester that fixed carbon in a long-lived marine reservoir. This strategy would contribute to a portfolio of climate mitigation efforts. To guide decision-making around testing, deploying, and regulating mCDR, we need to better understand how the deep sea and the broader Earth system would respond to increased biomass addition. The central processes driving this response are sensitive to choices about biomass type and storage site, and they stretch across spatial and temporal scales from microns to kilometers and from minutes to millenia. To organize this immense interdisciplinary challenge, we define five generalizable phases of a biomass-based mCDR project: inputs, placement, short-term response, long-term response, and functional stability. Each phase is associated with high-priority research objectives that could be achieved through thoughtful integration of direct field measurements, investigations of analog sites, experiments, and/or models. In-situ and laboratory experiments can be particularly powerful for isolating key processes; for example, in-situ “closed-system” bottle incubations can amplify small signals and reduce uncertainties created by complex physical flows. Regardless of approach, the overarching goal of biomass-based mCDR research is to develop a process-based understanding of biomass sequestration that is robust enough to project the likely outcomes of alternative choices related to mCDR. Beyond assessing carbon storage and ensuring regulatory compliance, future field experiments should prioritize generating the data required to improve models for impacts of biomass-based mCDR on the deep ocean at climatically-relevant scales.