Adoption Support, which works in the West Midlands and has six staff, will continue to operate all existing services from its current offices, but will work under the After Adoption name, the two charities said. There will be no redundancies.
Julie McBride, manager of Adoption Support, will retire, and the organisation will continue under Lynn Charlton, chief executive of After Adoption. The board of Adoption Support will step down, and the united charity will continue under the After Adoption board.
After Adoption has 68 staff and had a turnover of £2.7m in the last financial year, compared with £196,000 for Adoption Support.
Charlton said: "We have worked closely with Adoption Support throughout our history and are pleased to be able to pool our collective experience. Adoption Support has done some groundbreaking work and we are encouraged that the experienced team want to continue to work with us and share their knowledge.
"By consolidating resources and experience in this manner, it is hoped that we are able to have a greater impact on more people for as long as is needed."