"Marseille is the second city in France - it should have a great rugby club.Now the club are giving itself means to fulfil their ambitions," Lomu told the club website.
"Promotion to Pro D2 is the priority for the club this year. It's this challenge that pleased me - my ambition? Help Marseille become one of the great French clubs."
"The signings they have made show the club's desire to succeed and I want to be part of this adventure.
"My experience can help the club grow. I want to pass on what rugby has given to me not only to my team-mates but also to the club's youngsters."
Lomu was last seen in Europe playing for Cardiff Blues, but many thought he had finally retired in 2007. But the 63 times capped wing is having none of it.
"I never quit rugby. I decided to take a sabbatical year to be there during my partner's pregnancy, to experience this moment which is so important in a man's life," he added.
"Everyone thought that I had stopped my career, but it was a mistake. I always feel the need to play and only I will decide when it will be the end."