Combine radishes and cilantro in a heatproof quart container or large bowl.

Combine remaining ingredients in a medium saucepan and bring to a boil over medium heat, stirring until the sugar and salt dissolve.

Once the mixture comes to a boil, remove from heat and pour over radishes. Let sit at room temperature for 20-30 minutes or until it is cool enough to handle. Cover tightly with a lid and refrigerate for 1 day before using.

Radishes will keep covered in the refrigerator for up to 1 month.