As a result of the merger, Tamarisk, which is based in the London borough of Barnet and has 80 full and part-time staff, will in effect become the London branch of Self Unlimited.
Self Unlimited, which has 700 full and part-time staff, has operations in Newcastle, Preston, Leicester, Telford, Kent, West Sussex and Wiltshire.
The merger formalises an existing arrangement in which Tamarisk has been operating as an independent charity under the Self Unlimited umbrella.
A statement issued by Self Unlimited said "minimal redundancies" were expected.
Julie Riley, chief executive of the trust, will remain a member of the Self Unlimited board, a role she has filled since November 2009.
Patrick Wallace, who will remain chief executive of Self Unlimited after the merger, said: "It has been relatively easy for us both to make this transition because of the parity of our histories, operating ethos and charitable aims.
"We are confident that, together, we will offer a greater depth of expertise to a wider geographical area as we enter this next most important phase of our alliance."