Martinson left his role as director of news and PR at the Central Office of Information in 2011 and has since been acting as a consultant.
Before working at the COI, he was director of comms at the Food Standards Agency, head of media relations at BBC News and deputy head of press and publicity at the Millennium Dome.
He said: "I'm delighted to be joining Transparency International with its record of fearless campaigning and look forward to working with the team to have even greater impact."
