Armitage, 28, will link up with brothers Guy and Steffon as well ex-England international Jonny Wilkinson at the French club, with whom he has signed a two-year deal.
Speaking about the move, Armitage, who has made 165 appearances for Irish since making his debut in 2003, said he felt it was the right time to leave.
"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," he commented.
"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."
Irish head coach Toby Booth said: "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."