Strain crushed pineapple and reserve the pineapple juice that you squeeze out of the can.
Add pineapple juice and sugar to a medium saucepan. Cook over medium high heat for 1-3 minutes, stirring until the sugar dissolves.
Stir in cranberries and bring to a boil. Reduce to a simmer over medium-low heat until the cranberries burst and sauce reduces, about 10 minutes. (Sauce will thicken more as it cools.)