The company - a British subsidiary of French firm EDF - is one of the country's largest energy suppliers and will be a "tier one" sponsor of the games.
EDF will have exclusive marketing rights in the utility services sector for the London Olympics.
Financial terms were not disclosed.
"We're looking forward to working with EDF to make London 2012 a truly sustainable games, delivering a lasting legacy benefiting sport, the environment and the local and global community," London organizing chief Sebastian Coe said.
In March, London organizers signed up their first tier one sponsor, British bank Lloyds TSB, in a deal reportedly worth 80 million pounds (US$154 million; eur117 million).
EDF chairman and chief executive officer Pierre Gadonneix said his company was Europe's lowest carbon-emitting energy supplier.
"With these Olympic and Paralympic Games, we want to deliver the vision of inspiring future generations and leaving a lasting legacy, in sport and in society," he said.