Combine vinegar, water, sugar, and salt in a medium bowl. Whisk together until sugar and salt is fully dissolved.
Place onion in a jar or bowl (I like to use a mason jar). Pour vinegar mixture over the onions and let sit at room temperature for at least 1 hour if you're in a rush. Preferably, cover jar or bowl and place in refrigerator for at least 1 day. Pickled onions will last for a few weeks stored in the refrigerator.
Use pickled onions to top tacos, nachos, sandwiches, salads, and more!