Tickling is still poorly understood by science, but it doesn't appear that dogs are ticklish like humans. However, dogs do have a well-known reflex, called the scratch reflex, which triggers them to twitch a leg when touched in certain areas. This is popularly referred to as being ticklish.
The scratch reflex is a response to an irritant, and most dogs have it. Common areas that trigger it are the belly, neck, flanks and back. This reflex is usually not unpleasant for dogs, although it may bother some. Dogs may be especially sensitive to being touched in some areas, which is sometimes called being ticklish. Extreme discomfort or sensitivity to normal touch may indicate a health problem.