The global marine biogeochemical cycle of aluminum (Al) is believed to be driven by marine diatoms, due to the uptake of dissolved Al (DAl) by living diatoms from surface seawater. However, whether or not Al is incorporated into the structure of biogenic silica (BSi) of living marine diatoms is still unclear, which hinders the understanding of Al biogeochemical processes. In this study, visible evidence of structural Al in BSi of the living marine diatom, for the first time, was presented. The direct evidence is obtained based on the observation for the Thalassiosira weissflogii, a widely distributed marine diatom by focused ion beam (FIB) scanning electron microscopy and energy dispersive X-ray spectroscopy (EDS). The structural Al exhibited a homogeneous distribution in the BSi with the average Al/Si atomic ratio of 0.011. The fundamental results indicate the significant contribution of marine diatoms to the biogeochemical migration of marine Al.