In marine sediments, dissolved Mn is present as either Mn(III) or Mn(II). We apply a reactive transport model to geochemical data for a seasonally anoxic and sulfidic coastal basin to determine the pathways of formation and removal of dissolved Mn(III) in the sediment. We demonstrate a critical role for reactions with Fe(II) and show evidence for substantial benthic release of dissolved Mn(III). Given the mobility of Mn(III), these findings have important implications for marine Mn cycling.