ABSTRACTThe impacts of plastic, including carbon emissions and plastic pollution, have significant negative impacts on human well‐being and the environment. Recent research suggests that these impacts could be mitigated by using biomass to create products with lower carbon emissions or that reduce pollution through biodegradation or composting. As the scale of the plastic problem is substantial, the amount of biomass required for mitigation could be large. Biomass may have benefits, but it also has risks, including the potential to cause significant land‐use change. Land‐use impacts are widely acknowledged in the literature on plastic mitigation but are often downplayed with assumptions that changes in policies, behaviors, agricultural productivity, and technology can ameliorate the most negative impacts. This paper reviews the assumptions made about land use in the literature on biomass‐based plastics and plastic alternatives. Current studies generally make optimistic assumptions about land‐use change or have limited ability to account for land‐use change impacts. These assumptions, including technological and agricultural advancement, along with idealized feedstock sourcing, minimize potential land‐use impacts. This paper demonstrates how reasonable projections based on the literature could require a considerable amount of biomass, equivalent to a 7%–13% increase in global crop demand in 2040. Further research investigating projections for biomass use and the assumptions in these estimates is required to better understand potential land‐use impacts from bio‐based plastic substitutes. This research is important for informing emerging policies, including the UN Treaty on plastic pollution. Establishing criteria and thresholds for the sustainability of bio‐based alternatives, as well as identifying potential negative outcomes, will be crucial to avoid setting out on a path with significant unintended and potentially unavoidable consequences.