Watermelon is safe for cats. Seedless flesh, small pieces, no rind — technically fine. The more interesting question is whether your cat will have any interest in it, and the answer for most cats is: probably not.
Cats are among the few mammals with a documented genetic mutation that eliminates their ability to taste sweetness. They have no functional sweet taste receptors. Watermelon's main appeal — its sugary flavor — is simply not something cats can perceive. If you offer your cat a piece and they sniff it and walk away, that's expected behavior.
Some cats are curious about the texture or the cold temperature on a hot day. Others might be interested because you're holding it. A few cats genuinely seem to enjoy it, probably for reasons that have nothing to do with the sweetness. But unlike dogs, who often go nuts for fruit, cats tend to be indifferent.
If you do offer some
Remove seeds first — they're a choking risk and in larger amounts can cause digestive upset. Skip the rind entirely; it's tough and cats don't digest it well. A few small cubes of red flesh is the right serving.
Watermelon is mostly water — about 92% — so it's hydrating and low in calories. For cats that don't drink enough water (common, especially on dry food diets), a bit of high-moisture food like watermelon can help marginally with hydration. Though for that purpose, plain cooked chicken or wet food is a more nutritionally meaningful option.
There's no harm in offering watermelon to your cat. If they eat a little and seem happy, fine. If they're indifferent, that's the statistically more likely outcome.