Place radishes in a canning jar.

Combine the water, sugar and salt in a small saucepan over low heat and stir until dissolved. Remove from heat and stir in vinegar and mustard seed.

Pour mixture over the radishes in the jar. Screw lid on jar and gently shake to combine.

Cool to room temperature and serve or refrigerate until chilled. These should be kept in the refrigerator and are best served within 4 to 5 days.