Lomu has been training regularly with the Auckland Warriors and is in good shape, and lighter than he has been for some time.
"As soon as I heard about the Help For Heroes cause, and the game, I knew it was something I wanted to be a part of. I can still remember coming to Wales to play in Ieuan Evans' Testimonial Game at Stradey Park and, of course, spending time more recently playing with Cardiff Blues," said Lomu.
"It will be great to team up with Scott Gibbs in the International Select XV back line and to have one more outing at Twickenham Stadium. There are some great names already signed up to this fixture and I'm really looking forward to being a part of it."
Lomu was regarded as the first true global superstar of the professional game and is still the all-time top try scorer at the Rugby World Cup finals with 15. In 2007 he became a member of the International Rugby Hall of Fame, along with Ieuan Evans.
"This is great news for the game and for the International Select XV. What a great opportunity Mums and Dads now have to take their children to Twickenham to see four of the greatest names of rugby union's professional era in action at the same time," said International Select XV Team Manager, Ieuan Evans.
"Lawrence Dallaglio, Martin Johnson, Scott Gibbs and Jonah Lomu would get into any World XV chosen over the past 15 years. They alone will be worth the price of admission on 20 September for what promises to be a highly competitive, and potentially bruising, occasion.
"I know I'm going to ensure that my children have got their autograph books with them at Twickenham on the day because if they can get those four to sign they will have a slice of rugby history to cherish for the rest of their lives."