Sometimes — with care
A small cube of seedless watermelon flesh is harmless for most cats — some genuinely enjoy the water content on hot days. Seeds, rind and any regularity are the issues.
Watermelon flesh is non-toxic to cats and is 92% water, which is why the odd cat develops a summer fondness for it. As with all fruit for obligate carnivores, the value is hydration and novelty, not nutrition — and the ~6% sugar content means moderation matters, particularly for the many indoor cats prone to diabetes. Seeds contain trace cyanogenic compounds and are a choking hazard at cat scale; the rind causes digestive upset.
One or two small cubes occasionally on a hot day. If your cat is diabetic, overweight, or has digestive issues, offer fresh water or an ice cube to lick instead. A cat “stealing” watermelon regularly is mostly learning that melon time means attention.
An ice cube in the water bowl, or a splash of water from water-packed tuna, delivers summer enrichment with zero sugar.
This article is general information, not veterinary advice. If your pet has eaten something potentially harmful or shows symptoms, contact your vet or an emergency clinic immediately. Full disclaimer.