Having a cat in your home is so great that you’ve decided it’s time to get another one. The more the merrier!
You might be thrilled by this idea, but your resident cat might not show the same level of enthusiasm.
Moreover, you might even see her being rather unfriendly towards the new kitten.
Let’s take a look at the main reasons why is your cat hissing at new kitten, and also at some ways to stop this behavior.
1. Territorial Aggression
We all know how territorial cats are.
Imagine what kind of shock your resident cat is going through. She’s used to having the whole house for herself, just like all of your attention and your time.
Suddenly, she has to share all this with another kitten. This is a situation where many cats will show territorial aggression by hissing at a new kitten.
You can see the example of a cat’s hissing in the video below.
Your cat wants the new kitten to know that that is her house and that she was there first. VCA Animal Hospitals explains how a resident cat might even chase the other cat that attempts to approach her area.
You’ll easily recognize a cat showing territorial aggression by her standing upright, having her ears forward.
How To Solve?
You should show understanding for your resident cat in these moments.
Make sure she has her own things and her own space. Don’t let her and the new kitten share a litter box: Instead, provide three litter boxes in different parts of the house to ensure they each have privacy when defecating.
Also, make sure your resident cat has her own toys and scratching posts, as well as separate food and water bowls.
Do your best for her not to feel like someone is invading her personal space.
2. Stress And Anxiety
Your cat might be hissing at your new kitten because she’s stressed and anxious.
According to Marta Amat and her associates  some of the main causes of stress in felines are environmental changes, a poor human-cat relationship, and a lack of control and predictability.
The arrival of a new household member – the kitten in this case – is a significant environmental change that can cause a cat to feel extremely stressed.
Having a new kitten at home is a huge change for a cat, and makes her feel like she’s losing control over the situation.
How To Solve?
Many cats will react negatively to any kind of change in their environment. Therefore, you should keep things normal as much as you can, so that the arrival of a new kitten will be the only new thing in your cat’s surroundings.
Make sure she’s having her standard routine, and also make sure to spend even more time with her than usual.
Your cat might also be afraid that she’ll lose you since the new kitten arrived at the household.
With time, she will learn that you have plenty of time and attention for both her and the new kitten.
3. Play Aggression
Your cat hissing at the new kitten might be a form of play aggression.
If your resident cat hasn’t exactly had a chance to play with her littermates or other cats throughout her life, she could show a desire for playing with the new kitten – by hissing at it!
Cornell Feline Health Center points out how learning to play appropriately is a very important part of the cat’s socialization. Cats learn what playing is supposed to look like by observing the reaction of other cats.
If your cat has lived alone for her entire life, she might not even know what the playing is supposed to look like.
You might also notice your resident cat stalking the new kitten, and having dilated pupils, and her ears pinned to the tip of her head.
How To Solve?
You shouldn’t let your cat show any kind of aggression towards the new kitten.
Of course, you also shouldn’t yell or punish your cat, since this can just make her run away from you.
Instead, you should distract your cat when she starts to hiss at the new kitten by giving her a toy or starting to play with her.
It’s a good sign your cat shows desire to play with the new kitten, but their play session should be completely safe, and shouldn’t include any form of aggression.
4. Competitive Behavior
There is a good chance that your cat sees the new kitten as its competitor.
This is especially likely to happen with two male cats that are more prone to aggressive behavior. Your resident male cat will want to show he is the dominant cat in the household.
Competitive behavior isn’t only related to the same-gender issue. Your resident cat might dislike the new kitten because they share the same traits, such as size, coat color, etc.
If this is the case, your cat might see the new kitten as a real threat, and might hiss at it to show who’s the boss in the house.
How To Solve?
Some hissing and other behaviors that show the dominance of the resident cat are even considered normal.
So, a hiss from time to time shouldn’t be worrying, and you might not even have to interfere in the interaction between your old and new cat.
However, you should still observe these two together to be able to intervene if you notice their competitiveness has gone too far.
The hissing should stop when your resident cat starts to get used to the new kitten’s presence.
One thing you can do is to feed these two together. This way, your two cats will share the time they spent together with something positive – a delicious meal!
You should also reward your cat every time she shows nice behavior toward the new kitten. Use some of her favorite treats to show her she’s doing just fine, and that she can still have her own space and a lot of your attention, even though the family got bigger for one furry member.
Interesting Read: How to Tell Which Cat is Dominant
So, why is my cat hissing at new kitten?
You decided to grow your family with a new, adorable addition, and it seems that everybody’s happy except your resident cat.
She might show this dissatisfaction by hissing at the new household member, which could happen because your cat feels like her territory is being invaded, or because the new kitten is making her feel stressed.
This could even be a form of play aggression for your cat, or her way of competing with the new kitten.
However, with time and proper handling of this new situation, I’m sure your resident cat will get used to the presence of the new kitten.
Not only will she get used to it – she will also start to enjoy the new company, and will realize she’s not alone at home anymore, even when her dear owners are away.
 Amat M, Camps T, Manteca X. Stress in owned cats: behavioural changes and welfare implications. Journal of Feline Medicine and Surgery. 2016;18(8):577-586. DOI, Retrieved July 18, 2023.