A bald spot on your dog's buttocks (or any other part of the body) can have various causes, and it's essential to have it examined by a veterinarian to determine the exact reason. Some potential causes of bald spots on a dog's bum include: Dogs can develop allergies to certain foods, environmental factors (such as pollen or dust mites), or fleas. Allergies can lead to excessive scratching and licking, resulting in hair loss and skin irritation. External parasites like fleas, ticks, and mites can infest your dog's fur and skin, causing itching, redness, and hair loss. Bacterial or fungal infections can affect the skin, leading to hair loss, redness, and sometimes lesions or scabs. Problems with the anal glands, which are scent glands located near a dog's anus, can cause discomfort and lead to excessive licking or biting, resulting in hair loss in that area. A dog may injure the skin on its buttocks through scratching, rubbing against rough surfaces, or other injuries that lead to hair loss. Certain hormonal imbalances, such as hypothyroidism or Cushing's disease, can contribute to hair loss in dogs. In some cases, dogs may develop compulsive behaviors like excessive licking or chewing, which can cause hair loss in specific areas.