Combine the sugar and water in a medium saucepan over medium-high heat. Continue to stir frequently until all of the sugar has dissolved. Remove from the heat and add the celery seed, cover, and steep for 1 hour.
Strain though a fine-mesh strainer into a heatproof container and place in the refrigerator, uncovered, until completely cool. Place in an airtight container and store for up to 6 months.
To use: Add 2 tablespoons of syrup to 1 cup soda water and stir to combine.