Clark joined Hypernaked (as the agency was known) in February 2013 and oversaw its rebranding back to Naked Communications. Under his tenure, the agency picked up clients including Virgin Atlantic and Danone.
Matthew Melhuish, the global chief executive of Naked, said: "We thank Ian for his contribution to the business over the past year."
Gladwell co-founded Human Digital in 2008 and is the agency’s chief executive. Since leading its sale to M&C Saatchi in 2011, he has integrated the two agencies and worked on clients including Transport for London and Direct Line.
Melhuish said: "Christian brings to Naked a unique combination of skills and experiences, a sharp digital focus and an incredible energy, which will help guide Naked to the next level.
"Christian rounds out Naked’s world-class chief executive team with Troy Kelley (ex-R/GA) in New York and Carl Ratcliff (ex-TBWA and Channel 5) in Sydney, already gaining great momentum."