Put the mustard seeds, peppercorns, and sliced radishes into a pint jar.
In a small saucepan over medium heat, bring the vinegar, water, sugar, and salt, to a low boil, stirring occasionally, until the sugar and salt is dissolved.
Pour the vinegar brine over the radishes, covering them completely. Top off with more white vinegar if needed to fill the jar.
Cap the jar and let it cool to room temperature, then refrigerate. It’s best to wait 2-3 days so that the flavors can develop. The pickled radishes will last in the refrigerator for several months.
Notes
Use any type of radish can be used such as pink or red, watermelon, or daikon.
Pink or red radishes will turn the vinegar brine pink during the pickling process.
The pickling process tames the bite of the radishes slightly.