Coastal and Ocean Acidification Stressors and Threats Research Act of 2021 or the COAST Research Act of 2021 This bill addresses the effects of ocean and coastal acidification on marine organisms and ecosystems. Specifically, the bill reauthorizes through FY2026 and revises the Ocean Acidification Program of the National Oceanic and Atmospheric Administration and the ocean acidification grant program of the National Science Foundation. The bill expands those programs, as well as the ocean acidification activities of the National Aeronautics and Space Administration (NASA), to include efforts to research and monitor the effects of coastal acidification.