Rinse the jalapeños, pat them dry, and trim off the stems. Slice crosswise into thin rings.
Pack the slices into a 2-cup jar.
Combine the vinegar, water, sugar, and salt in a small saucepan. Bring to a boil, stirring until the sugar and salt dissolve.
Pour the hot brine over the jalapeños until they're fully submerged and the jar is filled to the brim.
Let the jar cool, uncovered, until it's no longer hot to the touch, then seal with the lid.
Refrigerate until chilled. Ready to eat in as little as a few hours, or overnight for deeper flavor.