Combine sugar, salt, vinegar, and water in a saucepan.

Bring to a simmer over medium heat and cook until sugar dissolves, about 5 minutes.

Remove from heat.

Add bay leaf, fennel seeds, mustard seeds, and black peppercorns to the mixture.

Let the mixture cool to room temperature.

Place rhubarb spears into a clean pickling jar.

Pour the cooled pickling mixture into the jar, filling to the rim.

Cover the jar with a lid and refrigerate.

The pickled rhubarb can be eaten after a few hours, but the flavor improves after a few days.