Slice the onion in half and remove outer skin. Use a sharp knife to slice the onion into uniform, thin slices. You want the slices to be about ⅛-inch thick.
In a medium jar or bowl with a tight fitting lid, combine the vinegar, water, sugar, and salt. Stir until sugar and salt dissolve.
Add the red onion slices and stir, making sure the onion is covered.
Refrigerate for a few hours, or preferably, at least overnight to let the flavor meld.