Pepco's shares rose 27 percent, or $5.69, to close at $26.93. Exelon ended 0.8 percent lower at $34.72.

The three-member commission, which regulates power, gas and telecommunications in Washington, voted 2 to 1 for the merger after the companies agreed to concessions demanded by city leaders.

"We've made a number of commitments to customers in all of the Pepco Holdings utilities' jurisdictions - the District, Maryland, Delaware and New Jersey - and we look forward to getting to work to deliver those benefits to our customers and communities," Exelon Chief Executive Chris Crane said in a statement.

Exelon struck deals worth $430 million with utility regulators in Delaware, Maryland, New Jersey and Washington, the company said in the statement.

The merger announced in April 2014 was previously approved by the U.S. Federal Energy Regulatory Commission and other state utility commissions.

The Washington D.C. Public Service Commission, which rejected the deal in August, agreed to reconsider after Exelon pledged to increase its proposed investment in the district to $78 million from $14 million. The company also reached an agreement with Mayor Muriel Bowser and Washington agencies on how to allocate the funds, which included protections for residential ratepayers.

Pepco serves about 2 million customers in the District of Columbia, Delaware, Maryland and New Jersey. Exelon has about 7.8 million customers in Maryland, Illinois and Pennsylvania.

(Reporting by Susan Heavey and Diane Bartz; Editing by Andrea Ricci and Richard Chang)

By Diane Bartz