A fishy smell coming from your cat’s litter box or backside is usually not coming from the urine itself. Cat urine naturally has a strong, sulfur-based odor, but it doesn’t typically smell like fish. When you notice a distinctly fishy scent, the most likely source is your cat’s anal glands, not their bladder. That said, urinary tract infections and other health issues can change the way urine smells, so it’s worth understanding the difference.
Anal Glands Are the Most Common Source
Cats have two small sacs just inside the anus that produce a pungent fluid. This fluid is normally expressed in tiny amounts during bowel movements, but it can also leak when a cat is stressed, startled, or has an underlying issue like impacted or infected glands. Research published in the Journal of Ethology confirmed that cat anal sac secretions contain trimethylamine, the compound responsible for a classic “fishy” smell. The secretions are also rich in short-chain fatty acids like butanoic acid and propanoic acid, which add a rancid quality to the odor.
The key distinction: cat urine gets its characteristic smell from sulfur-containing compounds, particularly 3-mercapto-3-methylbutanol. These produce a sharp, musky, almost skunky odor rather than a fishy one. Anal gland fluid and urine have completely different chemical profiles. So if you’re smelling fish near the litter box, your cat may have leaked anal gland fluid while using it, or the fluid may have expressed onto fur around the tail and mixed with urine in the box.
Signs that anal glands are the problem include your cat scooting along the floor, licking under the tail excessively, or a sudden burst of fishy smell when your cat sits on your lap. If the glands become impacted or infected, you might notice swelling near the anus or your cat straining during bowel movements. A vet can manually express the glands and check for infection.
Urinary Tract Infections Can Change Urine Smell
While a UTI is less likely to produce a specifically fishy odor than anal glands, bacterial infections in the bladder can make urine smell unusually foul or pungent. Bacteria break down compounds in the urine and produce byproducts that create new, stronger odors. You might notice the urine smells sharper, more sour, or just “off” compared to normal.
UTIs in cats often come with other signs: frequent trips to the litter box, straining to urinate, producing only small amounts, crying while urinating, or urinating outside the box. You may also see blood in the urine, though it’s not always visible to the naked eye. Male cats with urinary issues deserve particular urgency because a blocked urethra can become life-threatening within 24 to 48 hours.
Cats also commonly develop feline idiopathic cystitis (FIC), a stress-related bladder inflammation that produces many of the same symptoms as a UTI but without bacteria. According to International Cat Care, FIC can cause blood in the urine and changes in urination habits. Encouraging your cat to drink more water helps produce more dilute urine, which is less irritating to the bladder lining and less concentrated in odor.
Diet and Hydration Play a Role
Fish-based cat foods can make urine smell fishier. This is straightforward: the compounds your cat takes in through food are metabolized and partially excreted through urine. If you recently switched to a salmon, tuna, or ocean fish formula, the change in urine odor may simply reflect the new diet. Try rotating to a poultry or meat-based food for a couple of weeks to see if the smell changes.
Dehydration concentrates urine, making any existing odor more intense. Cats that eat primarily dry food tend to take in less water overall. Adding wet food, placing multiple water bowls around the house, or using a cat water fountain can increase fluid intake. More dilute urine smells less potent and is also better for urinary tract health in general.
Kidney Disease and Diabetes
Advanced kidney disease changes how waste products are processed, leading to a buildup of uremic toxins in the blood. In cats with kidney failure, this typically produces a strong ammonia-like smell, most noticeable on the breath rather than in the urine. Oral ulcers and bad breath are hallmark signs. The urine itself may actually become less concentrated and less smelly as the kidneys lose their ability to filter effectively, which is why increased urination and thirst are early warning signs.
Diabetes can also alter urine odor, though the smell tends toward sweet or fruity rather than fishy. When diabetes progresses to ketoacidosis, a dangerous complication, the body produces acetone and other ketones that give both breath and urine a distinctive sweet smell. This is a veterinary emergency.
How to Narrow Down the Cause
Start by observing where the smell is strongest. Sniff near your cat’s tail area versus the litter box itself. If the fishy smell clings to your cat’s fur around the hindquarters, anal glands are the likely culprit. If the smell is primarily in the litter box and the urine looks darker than usual, cloudier, or pinkish, a urinary issue is more probable.
Consider recent changes. A new fish-based food, a stressful event like a move or a new pet, or a change in litter box habits all point toward different causes. Stress can trigger both anal gland expression and feline idiopathic cystitis, so a suddenly anxious cat might present with more than one issue at the same time.
A few practical steps to try at home:
- Switch away from fish-based food for two weeks to rule out diet as a factor
- Increase water intake with wet food, extra water bowls, or a fountain
- Clean the litter box daily so you can monitor for changes in urine color, volume, or frequency
- Watch for other symptoms like straining, frequent litter box visits, scooting, or excessive licking under the tail
If the smell persists, appears alongside any behavioral changes, or your cat shows signs of pain or difficulty urinating, a vet visit is the logical next step. A urine sample and physical exam can quickly distinguish between anal gland problems, infection, and other underlying conditions.