The merger will allow Hope for Justice to expand its efforts of tackling modern day slavery to cover four continents: Europe, Africa, North America and Asia.
Hope for Justice said combining the charities will enable it to reach vulnerable children who are trafficked and targeted by predatory gangs who force them into sexual exploitation, domestic servitude and forced labour.
CEO of Hope for Justice, Ben Cooley said: "This integration gives us the ability to help more adults and more children who have been walking through some of the darkest days - to give them hope and to give them justice.
"There will be challenges to overcome and hurdles to clear, but we will go forward in strength and unity, because freedom is worth the fight."
Retrak runs transition centres, community programmes and educational/outreach 'stay safe' initiatives in several African countries. It claimed there's growing evidence of trafficking among the street children it works with.
Sir Peter Fahy, from Retrak, said he's excited to make a bigger impact.
"It [the merger] brings together Retrak's great strength in preventative work with Hope for Justice's strengths in rescuing victims and structural reform," he said.
"Both charities have strengths in working with law enforcement agencies and state bodies but, at the heart of both organisations, is the belief in reintegration through a process of support and restoration."
For the time being, the two charities remain legally separate organisations, but the trustees and directors have begun a long-term transition programme aimed at achieving a full merger.