Yes it is. In chocolate there is a substance called theobromine that we can digest with no problem, but dogs have problems to digest it so it becomes toxic because of high levels.
A small amount of chocolate will probably only give your dog an upset stomach with vomiting or diarrhea. With large amounts, theobromine can produce muscle tremors, seizures, an irregular heartbeat, internal bleeding or a heart attack. The onset of theobromine poisoning is usually marked by severe hyperactivity.
As the others have said the reason why chocolate is harmful to dogs I just want to add that it is also harmful to cats although they are less likely to eat it they should never be fed chocolate! (Mice and rats too!)
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