An 8 wk old male Yorkshire terrier was presented with a 2 wk history of recurrent hypoglycemia, lethargy, and seizures. Investigations revealed a marked increase in blood ammonia, low serum cobalamin, and increased levels of urinary methylmalonic acid (MMA) excretion. No liver vascular abnormality was detected. The patient was diagnosed with methylmalonic aciduria due to cobalamin malabsorption. The patient responded well to parenteral cobalamin administration, and the urinary MMA levels normalized rapidly following instigation of treatment. Due to the suspected hereditary nature of selective cobalamin deficiency, one sibling of this dog was screened and found to be normal. This is the first reported case of MMA secondary to hypocobalaminemia in Yorkshire terriers, and the second report of this disease in a dog in the United Kingdom. Given the fact that clinical signs of MMA are similar to those seen in dogs with portosystemic shunts and that Yorkshire terriers are predisposed to liver vascular abnormalities, this case report adds important clinical information to the current available literature.