Applications of the mineral olivine are a promising means to capture carbon dioxide via coastal enhanced weathering, but little is known about the impacts on important marine phytoplankton. We examined the effects of olivine dissolution products on species from three major phytoplankton groups: diatoms, coccolithophores, and cyanobacteria. Growth and productivity were generally either unaffected or stimulated, suggesting the effects of olivine on key phytoplankton are negligible or positive.