He thanked everyone for their support after raising more than £1 million for Sport Relief.
The 54-year-old completed his gruelling challenge, a tribute to the 27 years Nelson Mandela spent in prison before becoming the first black president of South Africa, by running two marathons on Sunday.
Izzard crossed the final finish line under a giant statue of Nelson Mandela in Johannesburg to cheering, applause and chants of "Eddie, Eddie, Eddie".
"It's been the hardest thing I've ever done and I'm very tired," he told the BBC.
"Thank you to everyone who has donated. This was tough so don't do this at home."
He celebrated moments later by swigging Champagne from the bottle.
Over the 707-mile route he battled through dehydration, heat exhaustion and sunstroke.
He was forced to take an unscheduled rest day on the fifth day of the challenge, which meant he had to complete two marathons on the final day.
The actor attempted the same feat in 2012 but was forced to stop due to health issues after marathon number four.
Izzard said: "This has been a long project but with the help of 'Team 27' I did it. Thank you so much to everyone who donated.
"I always knew I'd be back. When I didn't make it in 2012 I knew I'd be back. I can't stand upright, I have a huge blister and I'm exhausted. But I'm delighted."