The Association of Chief Executives of Voluntary Organisations and the National Council for Voluntary Organisations – which are based in the same building in London's King's Cross – made the announcement in a joint press statement.
A spokesman for ACEVO said that PR and comms activity would be part of the agreement.
"NCVO and ACEVO have agreed to work together to promote the positive impact of the voluntary sector. The decision was taken at a meeting last week of the chairs, vice-chairs and chief executives of the two umbrella organisations. This work is now under way," the statement said.
It went on to say that their respective memberships had "been telling us that they would like us to work together to make the case for a strong charity sector, especially with the risk of some negative stories about charities unfairly affecting the sector as a whole".
The statement ended by saying that the two organisations' CEOs would now flesh out a framework to "co-ordinate several projects" and develop new initiatives.
Previously, the two organisations have clashed publicly over various topics including charities' compliance with the lobbying act, and NCVO proposals last year to ensure the political neutrality of charities.
The ACEVO spokesman said it was "no surprise" that the groups should come together following what had been a difficult climate for the sector in political and media terms in recent months – which has included concerns over fundraising techniques, and the very public fall from grace of Kids Company.
The news was welcomed by the steering committee of the cross-sector Understanding Charities Group, which said in a statement: "Negative stories over the summer have underlined the importance of creating a shared narrative about the importance of charity and a co-ordinated response. We look forward to working with both ACEVO and NCVO on taking this forward."