Yes, existing iPhone 5s cases are compatible with the iPhone SE.
Most iPhone 5 cases will also fit on the iPhone SE. The only exception to this are older iPhone 5 cases, which may have different cutouts around the camera that are smaller than the pill-shaped True Tone flash. Unlike the iPhone 5s and iPhone SE, the iPhone 5 has a circular flash.Apple has also released an official leather case for the iPhone SE. This product is fully compatible with the iPhone 5, iPhone 5s and iPhone SE.