The three-member D.C. Public Service Commission voted unanimously on Tuesday to reject the merger. It had already been approved by several surrounding states.
Chicago-based Exelon announced in April of 2014 that it was buying Washington-based Pepco Holdings Inc. for $6.8 billion. The deal would create a large electric and gas utility in the mid-Atlantic region, serving about 10 million customers.
The companies were able to reach settlements with opponents of the deal in other jurisdictions, but not in the nation's capital.
Commission chairman Betty Ann Kane says the companies did not meet their burden of showing that the proposed merger would benefit the public.