Dummer or Dümmer may refer to:

• Dummer, Hampshire, a parish and village in Hampshire, England
• Dummer, New Hampshire, a town in Coos County, New Hampshire, United States
• Dummer, Saskatchewan, a hamlet in Caledonia No. 99 Rural Municipality, Saskatchewan, Canada
• Dummerston, Vermont, a town in Windham County, Vermont, United States
• Dümmer, a lake in southern Lower Saxony, Germany
• Dümmer See, a lake in Mecklenburg-Vorpommern, Germany
• Dümmer, Mecklenburg-Vorpommern, a municipality in the district of Ludwigslust, Germany
• Douro-Dummer, a township in central-eastern Ontario, Canada
• Fort Dummer, a British fort built in 1724 in Vermont

The Dümmer See or Dümmer is a large lake in southern Lower Saxony (Germany). It has a surface of 13.5 km² and an average depth of one metre. Its elevation is 37 metres.