Check four things first: your cat is eating normally, there is no vomiting, blood, or weight loss, the soft stool is recent, and a vet has seen any kitten, senior, or unwell cat. If all four are clear, a small amount of pumpkin fiber mixed into wet food is a reasonable trial.
Cats are not small dogs
Most pumpkin advice online is written for dogs, and cats need a slightly different lens. Cats are obligate carnivores with a shorter digestive tract, and they are notoriously sensitive to changes in their food. They also hide illness well. A cat with soft stool may have a simple diet issue, or it may be the first visible sign of parasites, inflammatory bowel disease, food sensitivity, hyperthyroidism in an older cat, or an infection.
The most important feline difference is appetite. A cat that stops eating for more than a day or two is at risk of a serious liver condition called hepatic lipidosis, especially if overweight. That makes "she has diarrhea and is not eating" a call-the-vet situation rather than a try-pumpkin situation.
Cornell's Feline Health Center has a clear owner guide to diarrhea in cats, including the causes vets consider and the signs that need prompt attention.
A pre-scoop checklist
If you reach the end with no stop signs, a short fiber trial is a sensible next step.
How pumpkin fiber works in a cat's gut
Pumpkin carries both soluble and insoluble fiber. The soluble part absorbs water and forms a gel, which can firm up loose stool. The insoluble part adds bulk and helps material move at a steady pace. Gut bacteria also ferment some of the fiber into short-chain fatty acids that feed the cells lining the colon.
For cats, fiber also has a reputation for helping hairballs pass, since bulkier stool carries swallowed fur through the gut. That is a side benefit to keep in mind rather than a reason to start. Water matters here as well: fiber pulls moisture into the gut, so a cat on dry food who drinks little may do better with more wet food in the mix during a trial.
The flip side is that too much fiber, or fiber introduced too fast, can cause gas, bloating, and sometimes even looser stool for a few days. Cats need small amounts.
Choosing and mixing the product
Plain canned pumpkin, not pie filling, works for some households, but an opened can spoils within a few days, and cats need only a tiny portion. A dry pumpkin powder keeps longer and is easier to measure.
Super Snouts Firm Up lists two ingredients on its product page, USA grown pumpkin and USA grown apple fiber, and describes itself as suitable for dogs and cats of all ages. The page gives a weight chart starting at one to two teaspoons for pets from 1 to 20 pounds, which covers most cats. For a cat, many owners start well below that, at about a quarter teaspoon, and build up over several days while watching the litter box. The page does not publish fiber grams per teaspoon, so stool quality is your guide.
Mix the powder into wet food with a splash of water so it blends in rather than sitting on top. Most cats ignore a small amount mixed well; some notice and refuse. If your cat eats only dry food, try a spoon of wet food as a carrier, or blend the powder with a little water and drizzle it over the kibble.
Never force a cat to eat a supplemented meal. If your cat refuses, remove the powder and serve plain food. Skipped meals are a bigger risk than soft stool.
What to watch during the trial
Track three things for one to two weeks: stool firmness, appetite, and litter box frequency. A simple note on your phone each day is enough.
Signs the trial is going well include firmer stool within a few days, a steady appetite, and no new straining.
Signs to stop and check with your vet include refusing food, vomiting, straining in the litter box, blood in stool, lethargy, or no improvement after about a week. A male cat straining in the box needs urgent attention, since straining can look similar whether the problem is constipation or a urinary blockage.
Recurring soft stool in a cat is usually telling you something. Common culprits include a food that does not suit her, frequent treat changes, parasites picked up outdoors or from a new housemate, and chronic gut inflammation. Your vet may suggest a fecal test, a diet trial with a single novel protein or a hydrolyzed food, or bloodwork for an older cat.
Fiber can be part of the long-term answer for some cats, but it works best once the underlying cause is known. Keep your daily notes from the trial and bring them along, since a record of stool, appetite, and timing shortens the path to an answer and helps your vet decide which tests to run first.
Cat and pumpkin questions
How much pumpkin can a cat have?
Start with about a quarter teaspoon of powder or a teaspoon of plain canned pumpkin per meal, and adjust slowly. Your vet can suggest a target amount for your cat's size and diet.
Can pumpkin help with constipation in cats too?
Fiber helps some constipated cats by adding bulk and holding water. Cats who strain repeatedly or produce nothing need a vet, since the cause matters.
Is pumpkin safe for kittens?
Kittens with diarrhea can dehydrate quickly and often have parasites. Check with your vet before adding anything to a kitten's diet.
Will my cat eat pumpkin?
Many will accept a small amount mixed into wet food. Some refuse anything new, so mix carefully and start with a tiny portion.
Can my dog and cat share the same pumpkin product?
They can share a container if the product is labeled for both, but each animal needs its own amount based on weight. Keep a separate scoop to avoid mix-ups.
Related reading: Pumpkin vs. Psyllium vs. Beet Pulp: Which Fiber for Which Problem; How much Firm Up does your dog need? A pumpkin-fiber feeding guide; Apple Pectin vs. Apple Fiber on a Dog Supplement Label: Are They the Same?