The merger, which will create a charity with a combined annual group income of £14m, will allow Street Child to have a greater impact and reach more children in poverty across the world, a spokeswoman for the charity said.
Street Child was set up in 2008, while Children in Crisis was founded by Sarah, Duchess of York, in 1993 and worked across Sierra Leone, Liberia, Democratic Republic of Congo and Afghanistan.
Children in Crisis, which had 16 staff, will operate under the branding of Street Child, which had 24 employees.
The new organisation is recruiting two senior roles – a head of UK and global philanthropy and a PR and content manager.
As part of the move, Street Child has moved into Children in Crisis’s offices in Stockwell, south London.
Tom Dannatt, chief executive of Street Child, will be the chief executive of the merged organisation, and Koy Thompson, chief executive of Children in Crisis, has left the organisation.
Anthony Wallersteiner, chair of Children in Crisis, will chair the merged charity and has been joined on the Street Child board by four trustees from his former charity.
The Duchess of York will remain a patron of the new charity, and her daughters Princesses Eugenie and Beatrice, will be global ambassadors for Street Child.
Dannatt said: "We have had experience of working with Children in Crisis on the ground in Africa and had identified many synergies between the mission and work as two organisations.
"Rather than compete for the same resources, we have merged as one organisation that is more responsive and effective. Our work remains unchanged, except to say that together the scale and scope of our impact is greatly strengthened."