As well as Cosmopolitan, Witham worked across cosmopolitan.co.uk, digitalspy.co.uk & sugarscape.com since 2014.
He begins at IBT Media on 7 March, responsible for "spearheading commercial growth across key markets".
IBT wants the new role to help drive growth geographically as well as across multiple disciplines and platforms.
Witham joined Hearst in 2001 when it was known as The National Magazine Company. He left in October when Duncan Chater took over responsibility for Cosmopolitan.
Dev Pragad, co-founder and managing director at IBT Media UK and EMEA, said: "Greg will be part of our core leadership team in London.
"With his skillset and confidence he’s best suited to help us become a leader in the cutting edge digital news business.
"His experience of identifying trends, driving growth and working across many platforms including print magazines and digital products makes him the ideal appointment."