Water chestnut — Food safety facts
Safe to share · Cats · full breakdown on the linked food page
Peeled and sliced, raw or cooked.
The full verdict, portion guidance and serving notes are collected on the Water chestnut food facts page.
Short answer
No, one small, peeled water chestnut is not an emergency and is safe for cats. They are non-toxic and low in calories, so a single piece won't harm your cat. Just make sure it was plain and not seasoned with garlic, onion, or salty sauces, as those are problematic. Keep an eye on your cat for the next few hours for any choking signs or unusual behavior, but serious issues are very unlikely from this tiny amount.