It will be devoted to any issues that falls outside the private or the public sector.
'The weekly will raise the profile of this sector and will reflect the professionalism and standing of the not-for-profit field and those who work in it,' said publishing director Rufus Olins.
The magazine will be part of Haymarket Business Publications, owner of weeklies including PRWeek, Campaign, Marketing and Planning as well as management titles such as Management Today and Human Resources.
Coverage in the magazine will reflect the achievements of the UK charity sector. The first issue under new ownership will be published on 13 March, with a new A4 format, design and editorial strategy.
Its readers will range from senior managers to fundraisers and team leaders among the UK's 170,000 UK charities, NGOs and voluntary organisations.
The magazine will provide an overview of the sector, as well as breaking news and providing analysis of relevant national news stories.
A team of 20 will work on the title. Existing editor Lucy Maggs remains in place, while Haymarket's Stovin Hayter, who previously launched digital economy business and marketing weeklyRevolution, will be editor-in-chief.
Existing staff on the magazine are being retained, while the ad team will be headed by Rachael Stilwell.
Third Sector was founded in 1992 by Arts Publishing International MD Martin Huber. As a fortnightly, the magazine has been pitched at senior executives in the voluntary sector. Huber will now concentrate on his core arts titles, but will work with Haymarket on a consultancy basis.