Its name and brand will remain the same and it will continue to take calls from children seeking help.
There will be a freeze on recruitment in both organisations in the hope of avoiding compulsory redundancies among the 230 ChildLine staff.
Mary Marsh, chief executive of the NSPCC, said agreement was reached on Monday night and the deal will take effect in February next year.
Esther Rantzen, founder and chair of ChildLine, will take a seat on the NSPCC trustee board and the ChildLine board will be dissolved.
Marsh said ChildLine complemented the There 4 Me service on the NSPCC website. NSPCC would also be able to offer some investment, training, back office functions and its network of supporters.
"It really is a case of one plus one equals three," she said. "People have been asking for years why we don’t work together.
"Technically it’s a takeover, but that gives the wrong impression of what it’s all about. They will retain their brand and continue to fundraise – we will take it on intact and continue to run it intact.
"They came to us for help, and earlier this year the trustees started talking. It was a brave decision and we have done it together." Marsh said ChildLine was about the size of the one of the divisions of the NSPCC, which has 2,000 staff and annual income of £115m.
Esther Rantzen said: "This is wonderful news for children. The NSPCC is a remarkable charity and we see ourselves as natural partners. When this move goes ahead, ChildLine will continue to provide exactly the same service. When children call 0800 1111 they will speak to a ChildLine counsellor who will give them comfort and support, just as they have for nearly twenty years."
ChildLine has depended more on spontaneous than planned giving, and has faced increasing problems. Last year it made 17 staff redundant and in July revealed a £1m shortfall and launched an emergency appeal.
The deal has been facilitated by the Charity Commission as part of its role in encouraging partnerships and collaborative working in the charity sector.
Andrew Hind, Chief Executive of the Charity Commission, said: "The NSPCC and ChildLine both have a long history of providing front line services for children who need help. The two have put the needs of their beneficiaries at the very heart of their decision making and have decided to work together.
"Where trustees have taken the decision to be involved in partnerships or joint working, we stand ready to help as part of our work to encourage innovation and effectiveness." For further information see www.ChildLine.org.uk, www.nspcc.org.uk or www.charitycommission.gov.uk.