Place the red onion slices into a jar and set aside.
In a glass or measuring cup, combine the vinegar, hot water, salt, and sugar. Stir until the salt and sugar are dissolved.
Pour the vinegar mixture into the jar, ensuring all of the onions are submerged.
Let sit for 40-45 minutes with the lid off to let cool. Once time is up, put a lid on the jar and store in the refrigerator.
Pickles are ready to eat at this point, but I recommend refrigerating at least 2-3 hours before serving when possible.