Malcolm Tyndall, director of communications and fundraising at Elizabeth Finn Care, said the decision to merge was taken because the organisations' strategies were so closely aligned.
Turn2us provides a free web service that helps members of the public to find grants, benefits and other financial help. Elizabeth Finn Care provides financial support to people in poverty.
"Both charities are going through a real period of expansion because there is so much demand," said Tyndall. "And there is an awful lot we can do better if we pool our services."
The move will increase the combined number of staff at the two organisations from 60 to 66.
Tyndall said Turn2us was originally set up as a separate organisation because Elizabeth Finn Care's charitable objects were not sufficiently aligned to the objectives of Turn2us. However, Elizabeth Finn Care has since amended its objects, making it possible to bring Turn2us under its control.
The two charities already share offices but will move to a new location in December.