Sesame seeds — Food safety facts
Safe to share · Cats · full breakdown on the linked food page
Plain sesame seeds are safe. Tahini is fine in tiny amounts.
Short answer
Yes, a single plain sesame seed is perfectly safe for cats. The seeds themselves are not toxic, and such a tiny amount won't cause any harm. Just offer your cat some fresh water and keep an eye out for any unusual behavior. If you notice vomiting or lethargy, give your own vet a call.