Heat oil in a deep-fryer or large saucepan to 375 degrees F (190 degrees C).
Whisk pineapple, flour, milk, and egg together to form a light and fluffy batter.
Bring reserved pineapple juice to a boil in a small saucepan. Add white sugar and stir to dissolve; lower heat. Simmer until thickened glaze forms, about 10 minutes. Remove from heat and cool to room temperature, about 15 minutes.
Drop spoonfuls of batter in the hot oil, 2 to 3 pieces at a time, and fry until golden brown, 3 to 4 minutes. Drain on paper towels.
Drizzle glaze over fritters and dust with confectioners' sugar.
