The 28-year-old has decided to make the jump from the Aviva Premiership to France after agreeing terms on a two-year deal at the Stade Mayol.
"I feel it's the right time for me to move on at this stage in my rugby career," Armitage told the club's official website. "Irish has been a huge part of my life and it's been a real honour to be a part of. I would also like to thank the supporters and team mates that I have got to know over the last decade."
London Irish head coach Toby Booth added: "Delon is one of many players who has committed over 10 years' service to this club. I wish him and his family the best in the future and thank him for his considerable involvement in London Irish's success in recent times.
"We are sad to see him leave but wish him the very best of luck at Toulon. Personally I hope he regains the form to see him return to international status in the future."
Armitage was axed from the England Saxons squad in January after he was arrested on suspicion of assault.