Work on excavating around 135m of new passenger tunnels will start next year and is due to be completed in 2027.
The station upgrade forms part of the wider redevelopment of Elephant & Castle, including the new shopping centre project being built by Multiplex for developer Delancey.
Deputy Mayor for Transport, Seb Dance, said: “I’m delighted TfL has now appointed a tunnelling contractor so work can continue on the vital Elephant and Castle Tube station upgrade, which will give the station step free access for the first time and deliver a new Northern line entrance and ticket hall.
“These significant upgrades will not only improve customer journeys through the station but will also support the delivery of thousands of new jobs and new homes in Elephant and Castle, supporting the Mayor’s aim for a fairer, greener London for everyone.”