Armitage has made 165 appearances for the Exiles since making his debut in 2003, however he has now signed a two-year deal with the Top 14 outfit.
The 28-year-old has long been rumoured to be set to join the likes of Jonny Wilkinson and Simon Shaw at the French side - with Armitage having fallen behind Ben Foden and Mike Brown in the England set-up.
It is the second time Armitage has played in France, with both Steffon and himself having starred for Nice before moving to London Irish.
And Armitage is looking forward to the move, telling the club's official site: "I would like to thank everyone involved at London Irish, in particular Toby Booth, who I have worked closely with during my time at Sunbury.
"My partner Jemma, our children and I, have thoroughly enjoyed the environment at the club and will go away with some fantastic memories.
"I feel it's the right time for me to move on at this stage in my rugby career. 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.
"I am looking forward to joining Steffon and Guy at Toulon but my immediate priority is ensuring that London Irish finish the season on a high with two victories."
Exiles head coach Toby Booth praised Armitage for all his efforts at the club.
"Delon is one of many players who has committed over 10 years' service to this club," he said.
"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."
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