Arriva has pledged to help TfL introduce new trains, modernise stations and run more frequent services as it takes on the new contract to run London Overgound.
Arriva will take over from the existing operator LOROL in November, and will support TfL in delivering further improvements under a contract worth £1.5bn.
The contract will cover seven and a half years, with an option to extend it for up to two additional years.
Gareth Powell, TfL’s chief operating officer, London Rail, said: "We have worked hard to deliver major improvements for our customers, taking a neglected part of the transport network and transforming it to support new homes, jobs and economic growth across the Capital."
"Arriva will build on this by extending operating hours, improving frequencies and introducing new trains.
"I would also like to thank the existing operator LOROL for their work that has helped make London Overground the huge success story it is today."
Customers on some routes will benefit from extended operating hours, and new services will be introduced on some routes.
Arriva will also be expected to deliver sustained improvements in performance levels, which will be supported with new incentives, which have been incorporated into the new contract. These include financially penalising Arriva should incidents caused by Network Rail, train and freight operators impact on London Overground services.
Arriva will operate all London Overground routes, including those that TfL took over last year from Liverpool Street station to Enfield Town, Cheshunt (via Seven Sisters) and Chingford, as well as services between Romford and Upminster.