Combine the water, vinegar, sugar, and salt in a small saucepan. Bring to a simmer and stir to ensure the sugar and salt are dissolved.
Place the jalapeños into a glass jar, such as a 2-cup Mason jar. Pour the hot pickle brine over top. Use a spoon to push the peppers down to ensure they are submerged. Let cool completely; then cover and stash in the fridge for many months.