We analyzed the biogenic silica (BSi) content and produced a diatom-based summer sea-surface temperature (SST) reconstruction for sediment core GC4 from the Holsteinsborg Dyb, West Greenland. Our aim was to reconstruct marine productivity and climatic fluctuations during the last millennium. Increased BSi content and diatom abundance suggest relatively high marine productively during the interval of AD 1000– 1400, corresponding in time to the Medieval Warm Period (MWP). The summer SST reconstruction indicates relatively warm conditions during AD 900–1100, followed by cooling after AD 1100. An extended cooling period during AD 1400–1900 is characterized by prolonged low in reconstructed SST and high sea-ice concentration. The BSi values fluctuated during this period, suggesting varying marine productivity during the Little Ice Age (LIA). There is no significant correlation between the BSi content and SST during the last millennium, suggesting that the summer SST has little influence on marine productively in the Holsteinsborg Dyb. A good correspondence between the BSi content and the element Ti counts in core GC4 suggests that silicate-rich meltwater from the Greenland ice sheet was likely responsible for changes in marine productively in the Holsteinsborg Dyb.