Whole boiled egg: yes, in cat-sized amounts
A cat can safely eat boiled egg, and unlike some human foods on the treat list, egg genuinely suits a carnivore. Both parts earn their place: the white brings lean protein, and the yolk brings fats, fat-soluble vitamins and a rich flavour most cats find irresistible. Boiling matters — thorough cooking removes the avidin problem that makes raw egg white a poor idea, because raw avidin blocks absorption of a B vitamin, and it also removes the food-safety worries attached to raw eggs generally.
Amount is the whole discipline. A typical cat needs only a fraction of an egg at a time: a quarter of a boiled egg, chopped, is a generous treat for an average adult, and even that should not appear every day. Egg is energy-dense, and a whole egg handed over as a snack quietly delivers a large slice of a small animal's daily calories in a form that, while nutritious, is not balanced as cat food.
Boiled eggs versus a cat's actual diet
It helps to understand why egg, for all its virtues, cannot replace cat food. A complete feline diet is formulated to include nutrients a cat cannot manufacture — taurine for heart and eye health among them — at levels tested for cats. An egg was designed by a hen for a chick, not by a nutritionist for a cat, so its profile is close and nourishing without being complete. Serve it the way you would serve a respected guest ingredient: on top of, or beside, the food that carries the nutritional load.
Preparation should stay boring on purpose. Boil until yolk and white are fully set, cool the egg before peeling, chop it into pieces small enough that a gulping cat cannot lodge a large wedge, and add nothing — no salt, no butter, no shell-on 'natural' experiments unless your vet has specifically discussed shell as a calcium source for your cat. Fried and scrambled eggs cooked in oil or butter are richer, greasier versions of the same food and are more likely to produce a loose stool.
Which cats should skip or limit egg
Most healthy adult cats handle a little boiled egg well, but introduce it gradually and read the results in the litter tray. Rich yolk can loosen the stool of a cat with a sensitive stomach, and a cat who needs to lose weight has no spare calories for daily egg, however wholesome it looks. Cats on prescribed diets — kidney, urinary or allergy-elimination plans — should not receive egg or any extra until the vet who set the diet agrees, because elimination and therapeutic diets only work when nothing else sneaks in.
A cat who has never eaten egg before should meet a few small pieces first, not a quarter egg at once. Watch for vomiting, diarrhoea, itchy skin or ear scratching over the following day or two; food sensitivities announce themselves in cats through skin and ears as often as through the stomach, and a reaction means egg joins that individual cat's no list.
Making 'occasionally' actually happen
Egg has a way of becoming a routine because the cat campaigns for it so effectively — few owners can resist an animal who appears at the sound of a shell cracking. Decide the household rule in advance, for example egg on two set days a week at most, and write it where feeders can see it. KinsLe Care Teams give that rule a home: feeding duties are assigned and marked complete with a timestamp, so whoever boils an egg on Tuesday leaves a record that stops a second helping arriving on Wednesday.