Prepare the rhubarb by washing, trimming the tough ends, and slicing the stalks into 1/2-inch pieces. Set aside.

In a small saucepan, combine the water, sugar, white vinegar, kosher salt, and peppercorns. Bring the mixture to a boil over medium-high heat, stirring constantly until the sugar and salt are completely dissolved.

While the brine heats, tightly pack the prepared rhubarb pieces into a clean, heat-safe pint-sized mason jar.

Once the brine is boiling and dissolved, immediately remove it from the heat. Carefully and slowly pour the hot brine over the rhubarb in the jar, ensuring the rhubarb is completely submerged. Pour until the brine reaches about 1/4 inch from the rim.

Gently tap the jar on the counter a few times to release any trapped air bubbles. Top off with a little more brine if necessary to fully cover the rhubarb. Seal the jar tightly with a lid.

Allow the jar to cool completely on the counter at room temperature for about 1-2 hours. Once cool, transfer the sealed jar to the refrigerator. The pickled rhubarb will be ready to eat after 24 hours, but the flavor develops best after 3 days.