The two organisations have announced that SKI UK will cease to exist from 1 April and its work will be merged into Plan UK.
SKI UK, which provides training for street children around the world to help them improve their lives, has three staff and had an income of £269,194 in the year to the end of June 2014.
Plan UK has 190 full and part-time staff and had an income of £63.2m over the same period.
Two of SKI UK’s staff will join Plan UK’s team, including the executive director Philippa Frankl, but there will be no redundancies because the third staff member was due leave the charity anyway.
One SKI UK trustee has joined the Plan UK board after going through Plan UK’s recruitment process.
The move came about after SKI UK approached Plan UK with a view to a merger in May last year, and both sets of trustees recently decided there was a good fit between the two organisations.
Tanya Barron, chief executive of Plan UK, said the move would help her charity to reach more of the most marginalised and vulnerable young people.
"It will also bring greater knowledge, skills and experience to our work around youth economic empowerment," she said.
SKI UK was established in 2008 as a sister organisation to Street Kids International, based in Canada. SKI closed in January and became part of Save the Children Canada.