The deal, which was announced yesterday, will involve Autism Plus retaining its own identity, structure, chief executive and board of trustees, and benefiting from opportunities to expand its support in the long term, a spokesman for the health and care charity Turning Point said.
There would be no job losses as a result of the agreement, he said.
He added that there would be long-term benefits for both organisations, with Turning Point bringing specialist knowledge in learning disabilities, substance misuse and mental health issues, and Autism Plus specific knowledge of supporting people with autism.
Turning Point has 2,775 staff and had an income of £112.3m in the year to the end March 2016; Autism Plus has 375 staff and had an income of £9m over the same period, according to figures on the Charity Commission website.
Philip Bartey, chief executive of Autism Plus, said: "With an ever-changing environment filled with challenges and opportunities, now is the time to build our capability to better meet the needs and aspirations of our customers through holistic joint working and collaboration.
"After detailed consideration and a due diligence process, the Autism Plus board recognised that Turning Point matched our culture and future growth aspirations, and I am delighted that we will now form part of a large force combining our strengths and solutions."
Lord Victor Adebowale, chief executive of Turning Point, said: "With Autism Plus now part of the Turning Point Group, we’ll form an impressive offer to those whom we support and those who commission our services.
"The spirit of social enterprise is embraced through the many social enterprises Autism Plus has created, a testament to the value of work and employment. These social enterprises will add to those already created by Turning Point.
"We are inspired by possibility and want to be able to empower more people to exceed expectations, realise their potential and transform more lives. With Autism Plus within the Turning Point group, we will work together to expand the reach of services from Autism Plus and the range of our offer."