The answer depends on which part of the plant
A small piece of ripe, red tomato flesh is not toxic to cats, and a cat who steals a cherry tomato from the counter will usually suffer nothing worse than a loose stool from an unfamiliar food. The rest of the plant tells a different story. Green, unripe tomatoes and the stems and leaves contain tomatine and related compounds in amounts that can genuinely harm a cat, causing drooling, stomach upset, weakness, and in larger amounts more serious signs. The rule is therefore about ripeness and plant parts, not about the word tomato itself.
Nutritionally, tomato offers a cat almost nothing. Cats do not need its vitamin C, since they make their own, and the flesh is mostly water and mild sugars. That makes tomato a food with real conditions attached and no real benefit, which is why it sits near the bottom of the worth-offering list even in its safe form.
Garden and kitchen are the real exposure routes
Cats rarely beg for tomato at the table. The common exposures are a houseplant-style tomato vine on a sunny windowsill, a garden cat chewing leaves while exploring, and a dropped green tomato during harvest. A curious indoor cat who mouths a tomato leaf once will often drop it because the taste is bitter, but repeated chewing or a kitten teething on stems deserves intervention rather than reassurance.
Move potted tomato plants out of reach, fence or supervise the garden patch if your cat roams it, and pick up windfall green fruit rather than leaving it in the grass. If you find chewed leaves and are unsure how much was eaten, watch closely for drooling, vomiting, diarrhea, unusual sleepiness, or wobbliness, and call your veterinarian or a poison advice line promptly if any of those appear — describing the plant part eaten matters more than the total amount.
Sauces, soups, and the seasoning trap
Cooked tomato products change the question entirely. Pasta sauces, soups, and salsas are typically built on onion and garlic, both harmful to cats, and often carry enough salt and spice to upset a stomach on their own. A cat licking sauce from a plate is not eating tomato in any simple sense, so keep plates out of reach and never offer sauce as a way to give a cat a taste of tomato. Plain ripe flesh, washed and offered deliberately, is the only form worth discussing.
Canned tomato also tends to be salted and acidic, and acidity alone can put a cat off eating afterward. Fresh and ripe is the narrow safe lane here, and even then a pea-sized piece is a taste, not a portion.
Recording a plant rule everyone can follow
Tomato risk in a home is mostly a communication problem: the gardener knows which plants are which, while the person letting the cat onto the porch may not. A written note attached to the cat's routine — tomato vines off-limits, ripe flesh harmless but not offered, sauces never shared — turns one person's knowledge into household practice. Note also which other garden plants you grow, since a vet call goes faster when the plant is already identified.
That kind of shared note is most useful at handover moments, when a helper is meeting the garden and the cat on the same afternoon. In KinsLe, placing the tomato vine warning inside Care Teams means the helper watering plants sees the caution before your cat follows them outside.