The Adolescent and Children's Trust (Tact) will run fostering and adoption services at Peterborough City Council, as well as supporting special guardianship carers across the city as part of a 10-year deal worth £126m.
The decision, which was rubberstamped this week, will see 36 council staff transfer to Tact. The permanency service is due to launch in April 2017, with the arrangement expected to deliver savings of £1m a year once fully established.
Staff working as part of the permanency service will be based at city council offices and overseen by a joint board that includes senior staff from the council and Tact.
The partnership will also provide an opportunity to develop new services for children and young people, including support for those returning home from care, the council said.
It is hoped the move will increase the number of city-based foster carers and reduce reliance on higher cost independent fostering and residential placements.
Sam Smith, lead member for children's services at Peterborough Council, said: "We believe that this new partnership will enable us to recruit and retain more local foster carers through an improved support network that Tact will deliver.
"This will lead to more local longer-term foster placements for Peterborough children and young people.
"As we reduce our spend on placements for children in care we will be able to invest in additional services for our most vulnerable young people.
"We believe this partnership will secure good fostering and adoption services for the long-term in Peterborough as council budgets are further reduced."
Andy Elvin, chief executive of Tact, added: "We are delighted to have been given the opportunity to work with Peterborough City Council to manage the new permanency service.
"We look forward to launching the service next year and further improving outcomes for children in Peterborough."
Earlier this year, former Education Secretary Nicky Morgan announced a strategic partnership between Norfolk County Council and children's charity Barnardo's to find innovative ways to improve outcomes for looked-after children and those leaving care.