The new firm, which will be operational from 1 November, has been created by private equity company Exponent following clearance by the Office of Fair Trading for Exponent's purchase of BBC Magazines for £121m.
It will be headed up by Tom Bureau, chief executive of Exponent-owned specialist publisher Magicalia.
Former EMI chairman Stephen Alexander will be chair of the company.
The new company will also include the magazine assets of Bristol-based publisher Origin, publisher of titles including Perfect Wedding and Craft Seller, and the digital and print assets of Magicalia, which include Practical Parenting and Junior Magazine.
Overall, Immediate Media will publish in excess of 50 titles.
As well as publishing a raft of magazine titles, Immediate Media will operate a long-term licence deal for 23 BBC Worldwide titles, including Gardeners' World magazines, BBC History and the CBeebies portfolio.
Immediate Media will employ 750 staff and will be based in both London and Bristol.
It is currently searching for a head office in west London.