The line has been closed for more than a month after storms at Christmas caused a landslip at Ockley.
More than 4,000 tonnes of new material have been put in place to rebuild the railway where the embankment collapsed, supported by a new wall of sheet steel, embedded in the ground.
The work was undertaken by Network Rail engineers and contractor BAM Nuttall.
A limited service from operator Southern had been operating over the undamaged track at peak times, but full service will now resume.
Network Rail’s route managing director Tim Robinson said: “This has been a fantastic effort from everyone involved. We have had teams on site since the line first collapsed on December 25 and a great deal of work had to be done just to get equipment to the location.
“We couldn’t have achieved that without the help and support of local residents, and we appreciate their involvement.
“We have also really appreciated passengers’ patience during the disruption and we thank them for their understanding. We will now begin the task of returning the work site to nature and removing the access road.”
A spokesman for Southern said: “We’re very pleased for our passengers that the line is now open. Network Rail has worked extremely hard under very challenging circumstances to repair the damage.”