Mix water, rice vinegar, sugar, and salt together in a microwave-safe bowl. Heat in the microwave for 1 minute and 30 seconds, stopping to stir every 30 seconds, or until sugar has completely dissolved. Continue warming and stirring for 30 seconds at a time, if necessary.

Place radish rounds into a canning jar. Pour vinegar mixture into the jar and top with a lid. Let pickle for at least 2 hours to develop the flavor.