Put water, sugar, and sliced bananas in a medium-size saucepan and bring mixture to a boil.
Once boiling, lower heat and let simmer for about 10 minutes, until mixture has thickened a bit.
Strain mixture into a bowl to remove bananas and any pulp. Press down on banana with wooden spoon to extract as much liquid as possible.
Keep syrup in bowl or pour into a mason jar and place in fridge to cool completely before using. Syrup will continue to thicken as it cools.