A mother cat singling out one kitten is alarming to watch, but it is information, not cruelty to punish or override. She may be detecting illness or weakness in that kitten that you cannot yet see, she may be in pain herself, or a first-time or stressed mother may simply be overwhelmed. The rejected kitten cannot wait while you work out which it is, so care runs in parallel: veterinary eyes on both animals, and immediate human care for warmth and feeding that does not depend on changing the mother's mind.
Understand the rejection before intervening
Note exactly what the mother does, because the detail guides the response. Moving the kitten out of the nest repeatedly, refusing to let her nurse while accepting others, growling, or biting at her are different from a mother who is simply distracted and accepts the kitten when you place her back. Check the rejected kitten for obvious problems you can see: a cleft in the roof of the mouth, a limb that moves differently, laboured breathing, or a body that feels cooler than her siblings. Check the mother too, for a hot, hard, painful teat, discharge, feverish lethargy, or distress when any kitten nurses.
Feed and warm the rejected kitten yourself
Until a vet advises otherwise, treat her as a hand-rearing kitten who happens to have littermates. Keep her warm first, because a cold kitten cannot digest food safely, using a covered heat source she can move away from, in a small nest placed near, but safely separate from, the main litter so she keeps their scent and sounds. Feed her on a regular schedule day and night appropriate to her age, tummy-down, letting her draw the feed at her own pace, and stimulate toileting after every feed. Weigh her daily and keep her record separate and unmistakable, because her progress, not the litter's, decides whether the plan is working.
Keep her socialized despite separate care
A hand-fed kitten still needs the social education littermates provide. Once she is warm, fed, and the mother is calm nearby, allow supervised time against her siblings for scent, play as she grows, and normal kitten wrestling, removing her if the mother becomes agitated or the kitten gets cold or exhausted. Handle her gently and often yourself, since human contact is now a larger share of her world, and let calm household sounds and routines reach her nest. This balance keeps her bonded to cats and people without gambling her safety on the mother's tolerance.
Get veterinary help without delay
Arrange a vet visit promptly for both the rejected kitten and the mother, the same day if the kitten is very young, losing weight, breathing poorly, or the mother shows any sign of illness or aggression that could injure her. Rejection is a recognised flag for problems in either animal, and early findings change feeding and care plans materially. Do not keep returning an aggressively rejected kitten to the nest unsupervised while you wait. KinsLe Care Teams helps you schedule her round-the-clock feeds, warmth checks, and weights with timestamps, so a rejected kitten gets devoted care without gaps between carers.