The DEC is an umbrella body for UK aid agencies and has run joint emergency fundraising appeals for international humanitarian disasters, such as the 2010 earthquake in Haiti, since 1963.
The DEC's membership criteria say that each member must have been registered as a UK humanitarian charity for at least five years, must have voluntary income of more than £4m a year and must spend more than £10m a year on overseas emergency work.
Marie Staunton, chief executive of Plan UK, said: "Membership will enable us to reach even more vulnerable children, especially girls, with vital protection and education provision when disaster strikes."
Clive Jones, chair of the DEC, said Plan's expertise and global commitment would help to make the body's response to future disasters more effective.
Membership of the DEC was capped at 15 places in 2008. A full review of members takes place every three years, with a part review taking place annually.