Measure 1 cup sugar, ¾ cup water, and ¼ cup Grand Marnier.
Combine the sugar and water in a small saucepan.
Heat over medium, stirring occasionally, until the sugar is completely dissolved and the liquid is clear.
Allow the syrup to come to a gentle simmer and cook 30–60 seconds to slightly thicken, then remove the pan from the heat.
Stir in the ¼ cup Grand Marnier until fully incorporated.
Let the syrup cool to room temperature, then transfer it to a clean, airtight container.
Chill in the refrigerator before use. Store refrigerated in the airtight container for up to 2 weeks.