The answer is both yes and no, as has been emphasized before, Marshmallows are high in sugar. This makes them rather undesirable to eat for longer periods of time for even a human.
For a dog, feeding them marshmallows would be equivalent to giving them poison, the carbohydrates that come from the sugar in the marshmallows could affect dogs badly, giving them food poisoning or worse, diarrhea.
Now, you might be thinking of giving your dog a sugar-free Marshmallow, don’t.
Sugar-free Marshmallows can be more harmful to a dog due to the compound they have, “xylitol”. This compound is especially dangerous for dogs as they actually promote liver problems and could be potentially fatal.
So, be sure to never give your dog a sugar-free marshmallow in the misunderstood hope that you are feeding them something tasty and healthy. Stay with a sugary marshmallow if you have to give it to your dog.