Barnardo's has signed a recognition agreement with Unison that grants the union sole rights to represent the charity's 5,500 staff.
Barnardo's has worked informally with Unison for many years on a number of issues. Late last year, union pension experts were invited to play a role when the charity made substantial alterations to its stakeholder pension scheme.
"We are pleased that we are able to announce the formalisation of our relationship with Unison," said Ian Theodoreson, Barnardo's director of finance and corporate services.
Unison's general secretary, Dave Prentis, has also welcomed the agreement.
"We look forward to working with Barnardo's to ensure they can continue caring for children and delivering quality services across the UK with well motivated staff."
The agreement follows months of negotiation and is one of the largest in the sector.