The 155-year-old organisation, which supports girls and women, said its former name "no longer stood for anything" and led it to be confused with YMCA England.
In a statement, Platform 51, which adopted the new name on 1 December, said: "Our new name more accurately represents who we are and what we do: 51 per cent of people are female, and girls and women use us as a platform to have their say and as a place from where they can move into the next stage of their lives."
The rebrand comes after the departure of Sarah Payne, its chief executive, who has left to become the first chief executive of the Wales Probation Trust.
She left on 30 November after two years in charge and will be replaced in January by Penny Newman, the former chief executive of the Jamie Oliver Foundation and Cafédirect, which markets fairly traded beverages.
Founded as the Young Women's Christian Association in 1855, the charity changed its name to YWCA England & Wales in 2002.
A spokeswoman for Platform 51 said the official launch of the rebrand would take place in the new year, once Newman had taken up her role.