Most thermoses are dishwasher safe, but verify the manufacturer's instructions first. Although some thermoses are dishwasher safe, the manufacturer may urge that you clean them by hand to maintain the material and surface. If your thermos is only meant to be hand-washed, you can probably clean it with water, dish soap, and a soft cloth or bottle brush. Fill the thermos halfway with warm water and a few drops of dish soap, then wait three to five minutes before washing or rinsing.