Why is my cat attacking my other cat? Causes and how to help

Miwuki illustration: Two cats in separate areas, each with its own mat and bowl

If your cat suddenly attacks your other cat, there is usually a specific trigger: something that frightened or aroused them and that they take out on their companion, an unfamiliar smell after a vet visit, pain, or tension over resources in the home. Separate them without using your hands, let them calm down in different rooms and book a vet appointment before assuming they “can no longer stand each other”.

Why does my cat suddenly attack my other cat?

A fight that seems to come out of nowhere almost always has a cause. These are the ones veterinary guides mention most often:

Redirected aggression. Your cat sees another cat through the window, hears a loud noise or is startled by something they cannot reach, and takes that arousal out on whoever is closest: their companion.
A different smell. After time away (the vet, a cattery, a trip), the returning cat may smell of the clinic or of other animals.
Pain or illness. Problems such as arthritis, dental disease or hyperthyroidism can make a cat more irritable.
Competition over resources and territory. Food bowls, litter trays, high spots or your lap can become a source of dispute, especially if there are few of them or they are close together.
Changes in the relationship. As they reach social maturity, some cats who used to get along start to clash.

Redirected aggression: when the anger is not about them

Imagine your cat is at the window, sees a stray cat in the garden and puffs up. Their companion walks past behind them and gets attacked. The target is not the real problem; they are simply the first cat within reach. Cornell recommends identifying and removing the trigger, for example by covering the lower part of the window if outdoor cats are the issue.

“My cat doesn’t recognise my other cat” after a vet visit

Cats identify each other largely by smell. International Cat Care explains that a cat returning from the clinic brings back unfamiliar scents that can unsettle the others and make it harder to recognise them as part of the group. It advises keeping the cats in separate rooms for a few hours or a day or two, then allowing gradual, supervised contact, and washing the bedding from the carrier.

Play or fight? How to tell the difference

Many cats, especially young ones, enjoy chasing and wrestling each other. Not every scuffle is a problem. Look at the whole picture:

Play: fairly quiet, with frequent pauses; roles swap (first one chases, then the other), claws do not cause injury and, afterwards, both go back to their routine as normal.
Fight: hissing, growling or yowling, puffed-up or side-on postures, one cat trying to escape while the other chases or corners them, and possible injuries.
Ongoing tension: even without open fights, a cat who stares at the other, blocks their way to the litter tray or pushes them off their spot is creating conflict.

If you are unsure, watch what happens next: after play, both keep sharing spaces; after a fight, one cat often hides or avoids certain areas.

What to do during a fight (without using your hands)

Never put your hands or any other part of your body between two fighting cats: you could be seriously injured. Instead:

Keep your distance and make a safe exit possible. Avoid shouting, clapping or adding other frightening stimuli.
Put a barrier between them, such as a large cushion, a board or a blanket, so they can no longer see each other.
Separate them into different rooms and leave them in peace. International Cat Care suggests 24 to 48 hours for both to calm down after an intense fight.
Do not pick them up or try to comfort them while they are still aroused: you could be on the receiving end of the next attack.
Check for wounds, limping or pain and call your vet if the fight was serious.

Punishing or shouting does not solve the cause of the conflict: Cornell warns that any physical punishment can increase a cat’s fear and make aggression worse.

How to calm a cat who is aggressive towards another cat: gradual reintroduction

When a fight has been serious or keeps happening, the most effective approach is usually to start from scratch, as if they were two strangers meeting for the first time:

Complete separation. Each cat in their own area, with their own food, water, litter tray, hiding place and bed.
Scent swapping. Swap their blankets between rooms, or gently rub one cat with a soft cloth and let the other sniff it.
Contact through the door. Feed each cat on either side of the closed door, at a distance where both eat calmly.
First sightings at a distance. Through a mesh barrier or a slightly open door, always pairing the other cat’s presence with food or play.
Short, supervised meetings that end before any tension appears. If you see hissing or staring, go back to the previous step.

Cornell notes that this process can take weeks to months. Moving slowly is, in the long run, quicker than forcing a meeting and having to start again.

If your cat hisses at the new kitten

An adult cat hissing at a newly arrived kitten is saying that the introduction is moving too fast. Keep the kitten in their own room for the first few days and follow the same steps of scent swapping and short meetings. If you are still preparing for the arrival, our guide to preparing your home before adopting a cat helps you set up that initial safe space.

Duplicate resources and a calmer home

Many conflicts ease when no cat has to compete for the basics. International Cat Care recommends:

Litter trays: one per cat plus one extra, in different places.
Food: several separate feeding stations, so no cat has to eat with their back to the other.
Water: several water bowls spread around the home and away from food.
Height and hiding places: shelves, tall scratching posts and hideaways where each cat can retreat.
Alternative routes: no important room should have a single entrance that one cat can block.

Daily play also helps channel energy, especially in young cats. And if you notice that one of them is hiding, avoiding the litter tray or changing their routine, review these signs of stress in cats and how to ease it.

When to see a vet or a behaviourist

Book a vet appointment if:

Aggression appears suddenly in a cat who used to be calm.
There are wounds, limping, swelling or any change in appetite or litter tray use.
Fights keep happening even after separating the cats and duplicating resources.
One of the cats is not neutered: both Cornell and the ASPCA list neutering as one of the first steps when there is aggression between cats.

Your vet will rule out pain or other illness and, if the problem continues, can refer you to a clinical animal behaviourist or a veterinary behaviour specialist. In some cases medication is considered, always prescribed by the vet and combined with changes to the environment. This guide does not replace a professional assessment of your cats.

Conclusion

A cat attacking their companion does not mean living together is a lost cause. Separating them safely, looking for the trigger, ruling out pain and reintroducing them without rushing solves many cases. The sooner you step in, the easier it will be for them to share the home calmly again.